Abstract
The Gran Canaria Blue Chaffinch is the rarest forest passerine in the western Palaearctic with a population reduced to a single locality, Inagua, in the west of the island. After a forest fire, some birds were established in another location and a reinforcement programme was launched with the aim of consolidating this new population. Between 2010 and 2019, 194 Blue Chaffinches were released in the pine forest of La Cumbre, in the centre of the island and at a higher altitude than the previous one. Birds of two origins were used for this action: captive-bred birds were released using “soft” release methods, and wild-caught birds from the Inagua source population were released using “hard” release methods. Survival of radio-tagged birds (20–42 days of battery life) was high and similar between the two groups. Wild-caught birds occupied 12–15 times more surface area than captive-bred birds that remained close to supplementary feeding sites. All translocated birds displayed breeding behaviour in the following breeding season, although the rate of contribution to the new population was significantly lower for wild-caught birds (21% compared to 54% for captive-bred birds). This disparity can be partly attributed to nearly one-fifth of the wild-caught birds returning to Inagua. We conclude that the establishment phase of the Blue Chaffinch reinforced program in Gran Canaria has been successful. To retain wild-caught birds and deter their return to the source population, we recommend employing methods akin to the soft-release techniques utilized for captive-bred birds. Ongoing monitoring of the enhanced population, collecting data on survival and breeding success, will enable the evaluation of the long-term success of the translocation program.
Zusammenfassung
Erfolgreiche Wiederansiedlung der aufgestockten Population des Gran-Canaria-Finken Fringilla polatzeki
Der Gran-Canaria-Fink ist der seltenste Waldsperlingsvogel der westlichen Paläarktis, dessen Population auf einen einzigen Ort, Inagua, im Westen der Insel beschränkt ist. Nach einem Waldbrand wurden einige Vögel an einen anderen Ort umgesiedelt und ein Programm zur Unterstützung und Stärkung dieser neuen Population gestartet. Zwischen 2010 und 2019 wurden 194 Finken im Pinienwald von La Cumbre, im Zentrum der Insel und in größerer Höhe als die der Originalpopulation, ausgewildert. Hierfür wurden Vögel zweierlei Ursprungs ausgewählt: in Käfigen aufgezogene Tiere, die auf schonende Weise ausgewildert wurden, und Wildfänge aus der ursprünglichen Inagua-Population, die mit „harten Methoden“ freigelassen wurden. Die Überlebensrate der mit Sendern ausgestatteten Vögel (20–42 Tage Batterielebensdauer) war hoch und bei beiden Gruppen ähnlich. Die in freier Wildbahn gefangenen Vögel besetzten eine 12-15mal größere Fläche als die in Käfigen aufgezogenen Vögel, die sich in der Nähe von extra eingerichteten Futterplätzen aufhielten. In der nächsten Brutzeit zeigten alle umgesiedelten Vögel Brutverhalten, obwohl der anteilige Beitrag zur neuen Population bei den Wildfängen deutlich geringer war (21% im Vergleich zu 54% bei den aufgezogenen Vögeln). Diese Diskrepanz lässt sich zum Teil darauf zurückführen, dass fast ein Fünftel der Wildfänge nach Inagua zurückkehrten. Wir schließen hieraus, dass die Einführungsphase des Programms zur Unterstützung des Gran-Canaria-Finken erfolgreich verlaufen ist. Um die in freier Wildbahn gefangenen Vögel zu halten und ihre Rückkehr in die ursprüngliche Population zu verhindern, empfehlen wir ähnliche Methoden wie bei den in Gefangenschaft aufgezogenen Vögeln, die behütet ausgesetzt wurden. Eine permanente Überwachung der verstärkten Population, bei der Daten zum Überleben und den Bruterfolg gesammelt werden, wird eine Bewertung des langfristigen Erfolgs des Umsiedlungsprogramms ermöglichen.
Similar content being viewed by others
Avoid common mistakes on your manuscript.
Introduction
Threats to wildlife can drive small and isolated populations to extinction. In particular, oceanic island species have suffered greater extinctions than continental species, with endemic species having a higher extinction rate than non-endemic species. Endemic species are restricted to a geographic range, sometimes associated with specialised environmental niches, limited dispersal capacity, small populations and reduced adaptive capacity. These factors make them susceptible to threats such as habitat loss, the impact of invasive species, overexploitation or the effects of climate change, reaching a threshold where stochastic factors, including Allee effects, amplify extinction risks (Frankham 1998; Duncan and Blackburn 2007; Chichorro et al. 2019; Lucas et al. 2019; Staude et al. 2020). Furthermore, Manes et al. (2021) emphasize the dire extinction prospects for island and mountain endemics under climate change. As the climate shifts, many species relocate to higher altitudes seeking climates more conducive to their survival (Walther et al. 2002; Walther 2010; Couet et al. 2022).
Translocation programs have become important tools in the conservation and management of endangered species, but their success depends on rigorous evaluation and testing, and the accumulation of further experimental evidence with different endangered species in different environments. Intensive management, like reintroduction programs, offers conservation benefits to threatened species, often averting potential extinctions (Armstrong and Seddon 2008; IUCN 2013; Seddon et al. 2014; McGowan et al. 2017).
Paramount to the translocation process is the methodology employed during reintroductions, with hard- and soft-release strategies each offering distinct advantages that can influence post-release survival and adaptation. The choice between hard- and soft-release methods in translocation projects remains a central discussion in conservation biology. Soft-release, involving a confinement period at the release site, often yields superior survival and reproduction outcomes due to enhanced acclimatization, especially in species with strong site fidelity or predation risks (Armstrong and Seddon 2008). However, it might present logistical and financial challenges. Conversely, hard-release, which involves direct release without acclimatization, is less complex and typically cheaper, yet may lead to increased initial mortality from predation or unfamiliar habitats (Teixeira et al. 2007). For hard-released wild birds, issues like disorientation, higher predation risks, and food resource challenges might arise. Therefore, translocation success can be improved by testing different methods, as the most effective technique depends on several factors. For example, Attum and Cutshall (2015) found that soft-release of translocated turtles was effective in continuous lotic habitats and hard-release being effective in patchy wetland complexes. Similarly, Cabezas et al. (2011) concluded that the survival of translocated wild rabbits was significantly influenced by predation risk and physiological health, advocating for soft-release techniques to minimize predation threats and guarantee optimal physiological conditions upon release. Supporting this, Wanless et al. (2002), working with the flightless Aldabra Rail Dryolimnas aldabranus, provided evidence that the soft-release is recommended as the conservative and precautionary method of choice for avian reintroductions and translocations, by allowing individuals to acclimatise and providing additional energetic reserves for the period between release and self-sufficiency. Captive-reared Burrowing Owls Athene cunicularia hypugaea had higher site-affinity, survival, and reproductive performance when reintroduced using a soft-release (Mitchell et al. 2011). However, a uniform consensus remains elusive. Hardman and Moro (2006) argued that soft release techniques do not necessarily confer an advantage to the successful immediate establishment and survival of hare-wallaby species in the short term. On the same line, De Milliano et al. (2016) showed that soft-release did not confer a consistent and substantive advantage for captive-bred Eastern Barred Bandicoots Perameles gunnii. Therefore, the success of soft or hard release is contingent on the natural history of the translocated species or the habitat type in which it occurs.
The source of individuals, whether captive-bred or wild-caught, introduces another layer of complexity in translocation programs. Animals for translocation can originate from either their natural habitat in source or donor populations, or from captive breeding centres (Seddon 2010; IUCN 2013). The nuances in behaviour, adaptation capacity, and survival rates between these two groups necessitate a comprehensive understanding. In conservation efforts, the use of captive-bred animals for translocations is frequently favoured, especially in scenarios where the action might exacerbate the vulnerability of a dwindling wild population (Wilson and Price 1994). Historical data show that wild-to-wild translocations are generally more successful than releasing captive-bred animals (Griffith et al. 1990). Nonetheless, these approaches are not mutually exclusive. As exemplified by Kraus et al. (2017) in their work with Eastern Hellbenders Cryptobranchus alleganiensis, integration of both strategies can be successfully achieved in translocation programs.
The success of a translocation program can be determined by two phases: the establishment phase and the long-term persistence phase. Studying the establishment phase is useful because reintroduced populations may fail in this initial phase under conditions that allow them to persist after the establishment phase (Armstrong and Seddon 2008; Kemp et al. 2015; Brockett et al. 2022). Reintroductions may fail in the establishment phase due to high dispersal or low survival while animals recover from the stress of translocation or acclimatise to the release site. Dispersal and survival of reintroduced birds in the immediate post-release period may depend on release protocols, particularly the acclimatisation time and number of birds released, as well as the provision of supplementary food and water in the release environment (Scott and Carpenter 1987; Armstrong and Seddon 2008; IUCN 2013; Bernardo et al. 2011; Armstrong and Wittmer 2011). Appropriate post-release monitoring provides the metrics and feedback mechanisms that inform the adaptive management of translocations, which will contribute to their ultimate success (IUCN 2013).
Therefore, in order to evaluate the reinforced programme in its establishment phase, it is necessary to monitor the translocated animals closely to know their movements and immediate survival. Animals must be identifiable to allow individual monitoring. For small birds, combinations of coloured rings are often used, although advances in radio telemetry have allowed continuous monitoring of smaller species, such as most passerines (Kays et al. 2015; Taylor et al. 2017). Telemetry has become an increasingly powerful and accessible means of collecting much of the detailed information needed to effectively evaluate translocation projects (Whitford and Klimley 2019), but the use of telemetry is a balance of costs and benefits where researchers must offset potential negative effects of capture and tagging by maximising the benefits of their study (Kays et al. 2015).
The Gran Canaria Blue Chaffinch Fringilla polatzeki is a mountain passerine endemic to Gran Canaria that inhabits Pinus canariensis forests (Lifjeld et al. 2016; Sangster et al. 2016). The main causes of the species' decline in the past have been the destruction and fragmentation of the pine forest as a result of unsustainable logging and the indiscriminate collection of specimens for natural history museums. Currently, the greatest threats to the species are considered to be the reduced extent of the pine forest, the effects of climate change and the destruction of habitat by large forest fires. At the beginning of the twenty-first century, the species was restricted to the Inagua Nature Reserve. In 2008, after a large forest fire in this area, a few birds were found breeding successfully in another pine forest, La Cumbre, possibly due to birds fleeing the fire and settling in a pine forest with optimal conditions for the species. Therefore, this site was chosen to develop a reinforcement programme (population augmentation), starting with birds bred in captivity and subsequently translocating individuals captured in Inagua as a source population (LIFE + PINZÓN 2019). Preliminary analyses give a positive assessment of the program, which started with birds bred in captivity and recorded survival and reproductive success values similar to those of natural populations (Delgado et al. 2016). Subsequently, Illera et al. (2023) found that the reinforced population at La Cumbre, from juvenile individuals from captive breeding and captured from the source population of Inagua, seemed to be viable in the short term, considering the similar values of biometric measurements, reproductive success, body condition, genetic diversity, and inbreeding of wild-born birds in both populations. As a result, the reinforced program led to a 16% growth in its population, highlighting the importance of such actions in safeguarding bird species with limited populations confined to a single location (Carrascal et al. 2022). The total population size of the Gran Canaria Blue Chaffinch is estimated at 430 birds, restricted to a very small area of 60 km2 (Carrascal et al. 2022), making it the forest passerine with the smallest population size in the Western Palearctic, and therefore, still listed as Endangered by the IUCN (BirdLife International 2021).
The aim of this paper is to evaluate the establishment phase of the reinforced program of the endangered Gran Canaria Blue Chaffinch. We investigated the home ranges of released juvenile chaffinches—both captive-bred at the Tafira breeding centre and wild-caught from the Inagua source population—using radio-tracking. Previously, we examined the impact of radio transmitters on the birds' short- and long-term survival rates. Finally, we estimated the contribution of both bird groups to the reinforcement of the translocated population, considering the birds found in La Cumbre during the subsequent breeding season after the release. The findings of this research could inform the species' management strategies and offer insights for other reintroduction efforts targeting endangered forest passerines on islands.
Methods
Study site
The island of Gran Canaria (1560 km2) is located in the centre of the Canary Islands (Spain). The characteristics of the habitat occupied by the Blue Chaffinch are described in Carrascal et al. (2017, 2022). The pine forests of Inagua, in the west of the island, host the main population of the species, which served as the source population in the reintroduction programme (LIFE + PINZÓN 2019). It is a mature forest of Pinus canariensis that develops under semi-arid conditions. Most of these pine forests are protected in the Inagua Integral Nature Reserve (37.6 km2). The pine forests of La Cumbre (21 km2), where the birds were released, are located in the centre of the island and at a higher altitude than the previous one. It is an immature pine forest, the result of reforestation carried out in the middle of the last century in the central part of the island. The two pine forests are separated by 5 km and are poorly connected by forest stands (Fig. 1).
Location of Gran Canaria in the Canary Islands, showing the location of the Captive Breeding Centre of Tafira (star), the sites where wild birds were captured in Inagua (black dots) and the sites where releases took place in the translocation area of La Cumbre (white dots). Pine forests are shaded in green. The two lower panels of the figure show the location of 30-min units survey in search of colour-banded birds in Inagua and La Cumbre (colour figure online)
Origin of translocated birds
The Cabildo de Gran Canaria managed a captive breeding centre in Tafira (Las Palmas de Gran Canaria), where there was a captive stock of breeding pairs in aviaries. The genotyped individuals used in the captive breeding programme had the same allelic diversity as the free-ranging birds in the wild source population, therefore, the released birds were genetically viable (Garcia-del-Rey et al. 2013). The captive breeding of blue chaffinches and release protocols are described in Díaz and Delgado (2021). In addition, wild birds were captured in Inagua, where the main population of the species remains (Carrascal et al. 2022). Captures were made using mist nets in late summer in places where the birds go to drink and bathe. Thus, between 2010 and 2019, 194 individuals of the Gran Canaria blue chaffinch were released in the pine forests of La Cumbre to strengthen the population. A total of 114 birds born in captivity and 80 translocated from the wild were released (see Supplementary Material S1). Ninety-three per cent of the birds released were juveniles born in the same year. The birds were released between the end of August and the beginning of October each year, when the young are able to feed themselves and are no longer dependent on their parents.
Wild-caught and captive-bred birds were released using different methods, informed by our primary focus on the conservation of this endangered species and the imperative to minimize juvenile mortality. Captive-bred individuals, which are costly to produce and accustomed only to aviary life, underwent acclimatization for 2–14 days in groups of 2–7 birds in 24–50 m3 aviaries located within the pine forest of their release site. This soft release approach allowed these birds to adapt gradually to their natural environment and the presence of other birds, while also providing supplementary food to ease their transition to the wild (De Milliano et al. 2016). Conversely, wild-caught birds were released using hard methods 9.5–14 km away from their capture site, after spending 24 h in transport boxes to mitigate stress. This method was chosen to minimize injury risks, such as damage to wings, beaks, and tails, which could occur in large aviaries during prolonged periods. Additionally, keeping them in darkness at night in small boxes allowed close monitoring of their condition, addressing the stress associated with capture and human contact. All released birds wore a combination of coloured rings that allowed individualised identification from a distance. Five supplementary feeding and water points were placed in the surroundings of the release site throughout the following five months.
Monitoring and telemetry
For this study, 49 individuals were fitted with radio transmitters before being released: 15 captive-bred birds (years 2013–2017) and 34 wild-caught birds (years 2015–2018). Model BD-2 devices (Holohil Systems, Ltd., Carp, Ontario, Canada) lasted approximately 20–42 days and weighed 0.75–0.82 g, in no case exceeding 3% of the birds' body weight (average juvenile weight = 27.4 g, sd = 1.73). A modification of the leg harness using 0.5 mm diameter absorbable surgical suture was used to attach the transmitters, so that the harness detached after a few months, minimising adverse effects on birds (Karl and Clout 1987). Two model R1000 receivers and a three-element Yagi antenna were used for monitoring. On a daily basis, two people carried out radio tracking and manually triangulated bird locations. We obtained at least one precise location of each bird per day and determined its status (alive or dead).
Between spring and summer of 2011–2020, we conducted intensive surveys and searches in the two populations to resight the released birds in 2010–2019, monitor their reproduction in subsequent breeding seasons, and observe other aspects outside the scope of this study. Surveys were conducted on foot along paths and cross-country tracks. In La Cumbre, surveys were throughout the whole pine forests, while in Inagua, due to its vast size, areas with less suitable habitat for the species were excluded (following Carrascal et al. 2017). From 2017 onwards, a more standardized survey approach was adopted, segmenting the effort into 30-min units and recording both initial and final geographic locations, providing a detailed spatial understanding of the survey effort: 2139 30-min survey periods were conducted in Inagua and 1526 in La Cumbre (Fig. 1; in 2011–2016, the sampling effort was similar although not georeferenced in detail).
Data analyses
Survival estimates were made with MARK (White and Burnham 1999) using “Live Recaptures (CJS)” and “Nest Survival” analysis. Spatial analysis of radio-tagged birds was performed with R software version 4.2.1. (R Core Team 2022) using the sp, AdehabitatHR and maptools packages (Pebesma and Bivand 2005; Bivand and Lewin-Koh 2023; Calenge and Fortmann-Roe 2023).
First, we examined the visual capture-resight history of coloured-ringed wild-caught birds from the springs and summers following liberation to test the effect of radio transmitters. We compared the annual survival of wild-caught birds released in La Cumbre during those years: 34 birds with transmitters and 28 birds without transmitters. Annual survival was estimated using MARK considering the effect of group (birds with and without transmitter), year of monitoring and the interaction between these two factors on survival (S) and resight probability (p). The Akaike Information Criterion for small sample sizes (AICc) was used to assess the fit of the models to the data, with those models with a difference in AIC (∆AIC) of less than two units considered to have a similar fit to the data (Burnham and Anderson 1998).
Second, we estimated the daily survival of the released birds with the radio-tracking data in La Cumbre for a maximum period of 42 days. In this analysis, we examined 15 captive-bred juveniles from Tafira and 34 wild-caught juveniles from Inagua, all released in La Cumbre between 2013 and 2018. The MARK models considered the effect of the origin of the birds (captivity or wild) and the time (days) after liberation. AIC values were also used to assess the fit of the models to the data.
Home range
The area occupied by radio-marked individuals during the study period was estimated using minimum convex polygons (MCP) for 95% of the locations, excluding 5% of the most distant locations. To obtain information on activity centres, home ranges were also estimated using kernel analysis with 95% encounter probabilities. The reference and least square cross validation (LSCV) smoothing parameter (h-bandwidth) was calculated to find the best kernel fit to the data (Seaman and Powell 1996; Fieberg 2007; Baíllo and Chacón 2021). To estimate home ranges, five birds with less than five locations were excluded (Pebesma and Bivand 2005). To avoid bias in the kernel activity centres due to heterogeneity in the number of contacts of each bird per day, the first location of each bird per day was used in cases where there were several locations per day. See Supplementary Material S2 for the home range areas of the studied birds.
Contribution of translocated birds to the reinforced population
Survival alone should not be used as a parameter for assessing the establishment phase of a translocation programme, as there may be birds that survive but do not contribute breeding individuals to the translocated population. This occurs when birds disperse widely or return to the area of origin. Therefore, as a surrogate measure of establishment success, we measured the number of translocated birds that remained in La Cumbre release area during the following breeding season, thereby contributing to the translocated population. The percentage of released birds sighted again in La Cumbre during the following breeding season was estimated for those coming from the captive breeding centre in Tafira or wild-caught in Inagua (using 10,000 bootstraps for each group of birds). The prospection of the pine forests of Inagua and La Cumbre in search of translocated birds (see Monitoring and telemetry section) allowed resightings and determined the dispersal/return rate of released birds during the reproductive period of the species. We also recorded whether the birds exhibited breeding behaviour (mating and territoriality).
Results
Survival analyses
The fate of six birds could not be determined because either the transmitter became detached, some birds disappeared and were not found, or the transmitters were emitting motionlessly from an inaccessible point, making it impossible to know whether the birds died or the transmitter became detached. Of the remaining 43 birds, 16 were found dead during radio-tracking. Causes of death were predation by a bird of prey (7 wild-caught birds; probably by Eurasian Sparrowhawk Accipiter nisus) and feral cats (2 captive-bred birds). Additionally, separate incidents involving wild-caught birds resulted in one death due to a collision with a vehicle on a road and another due to entanglement in a pine needle caused by a plastic ring. For the remaining birds (5), the cause of death could not be determined (Table 1).
Table 2 shows the model selection for survival of wild-caught birds with and without transmitters. There was a slight overdispersion in the global model explaining survival and resight probability, so a c-hat adjustment was made. The null model, excluding the effect of transmitters and year of capture, had the highest weight (0.372). The mean annual survival and resight probability obtained by averaging the values of the three most plausible models was similar between the two groups: 0.459 (SE: 0.074, 95% CI 0.321–0.602, N = 34) for birds with transmitters and 0.488 (SE: 0.075, 95% CI 0.346–0.633, N = 28) for birds without transmitters. Therefore, the wearing of a transmitter had no effect on the long-term survival of birds.
A daily survival analysis was performed for captive-bred and wild-caught radio-monitored birds. The time model was the most plausible with a weight of 0.574 (Table 3). Daily survival of both groups of birds during radio tracking, estimated by model averaging of the two best models with ΔAICc < 2, is shown in Fig. 2. Mortality occurred in the first few weeks after release, so daily survival increased with time. For example, on day 10, the estimated survival 95% confidence intervals were 0.967–0.992 for captive-bred juveniles and 0.969–0.993 for wild-caught juveniles, indicating comparable survival rates between the two groups during the initial post-release period. Therefore, the survival of transmitter-bearing birds, during the first 42 days of radio-tracking, was not affected by their origin, whether they were captive-bred or captured from the source population at Inagua.
Home range
A total of 993 locations were obtained from 14 captive-bred birds (372 locations) and 30 wild-caught birds (621 locations). The mean locations per captive-bred and wild-caught bird were 26.6 (range 5–38) and 20.70 (range 5–43), respectively. None of the individuals translocated from Inagua returned to the site of capture during radio tracking. The size of the pooled MCP95 was 142.25 ha and 907.10 ha for captive-bred and wild-caught birds, respectively (Fig. 3).
Home ranges during the 42 days of radio-tracking for the set of released birds. 95% Minimum Convex Polygons of released birds (A), where the larger polygon corresponds to wild-caught birds and the smaller polygon corresponds to captive-bred birds. For the 95% kernel estimation area (B), wild-caught birds are represented by the red area and captive-bred birds by the black area. The triangle indicates the point of release and the crosses indicate points of contact with released birds (colour figure online)
Home range (in logarithm), estimated by the MCP95 method (Minimum Convex Polygon 95%), was significantly related to the number of locations of each individual (b = 0.102, se = 0.020, P < < 0.001) and was higher for wild birds from the Inagua population than for captive-bred birds (P = 0.00015). This ANCOVA model explained 46.8% of the variance in home range (in logarithm) of the 44 birds studied. The adjusted mean of the home range for captive-bred birds was 2.4 ha (95% confidence interval 0.9–6.1 ha; total range 0.2–69.9 ha), while for wild-caught birds, it was 25.8 ha (95% CI 13.6–49.4; total range 0.1–549.1 ha). Repeating the analysis for those individuals with 18 or more records (12 captive-bred birds, 15 wild-caught birds), the effect of number of records per individual no longer reached significance (P = 0.236), while the effect of bird origin remained significant (P < < 0.001), with a mean of 5.5 ha for captive-bred birds (95% CI 2.3–13.2 ha) and 83.9 ha for wild-caught birds (95% CI 38.4–183.1 ha).
For kernel analysis, the most appropriate bandwidth was h-ref, as h-LSCV was unstable for estimating the home range of some birds due to its low value (see Supplemental Material S1). The 95% kernel home ranges were significantly correlated with the 95% minimum convex polygon estimates for those radio-tagged individuals with 18 or more locations (r = 0.82, P < < 0.001). Repeating the analyses with the 95% kernel estimates yields nearly the same results regarding significance patterns (lack of significance of the number of records per individual—P = 0.738—and strong effect of bird origin—P < 0.001—), and home areas: adjusted means of 6.8 ha for captive-bred birds (95% CI 2.7–16.8 ha) and 85.6 ha for wild-caught birds (95% CI 38.1–194.4 ha).
Contribution of translocated birds to the reinforced population
All translocated birds from 2010 to 2019 found in La Cumbre during the following springs and early summers showed breeding behaviour typical of the species (mating, singing males, fidelity and territorial defence, etc.). The contribution of captive-bred birds to the La Cumbre population was 52% (95% CI 42–61%; N = 114), higher than that of wild-caught birds, which contributed with only 21% (95% CI 12–30%; N = 80). This was partly due to the fact that 18% of the wild-caught individuals returned to the area of capture (i.e. they were detected in Inagua during the following breeding season). Very similar results are obtained when repeating the analysis with birds released during the entirely overlapping period of 2015–2019; captive-bred: mean = 47%, 95% CI 36–59%, N = 59; wild-caught: mean = 21%, 95% CI 12–30%, N = 80.
Captive-bred birds released in La Cumbre during early autumn nested, on average, 0.82 km away the following spring in the same area (95% CI 0.70–0.96 km; n = 51, all well-documented cases). In contrast, wild-caught juveniles from Inagua, released in La Cumbre, settled an average of 2.12 km from the release site in the subsequent spring in La Cumbre (95% CI 1.46–2.86 km; n = 13). The non-overlapping confidence intervals between these two groups of birds, each with clear indications of nesting behaviour, is consistent with the previous results regarding larger home ranges for Inagua's wild birds compared to captive-bred counterparts.
Discussion
Our research provides several key insights into the survival, movements, and contribution of translocated birds belonging to an endangered species. These findings can have important implications for the design and implementation of translocation programs, particularly for philopatric endangered woodland species that inhabit islands. Captive-bred birds liberated with soft-release techniques made a higher contribution to the reinforced population compared to wild-caught birds with hard-release techniques. This is a pivotal insight for translocation programs, as it might suggest the advantages of releasing captive-bred individuals over wild-caught ones in certain contexts. Wild-caught birds displayed larger home ranges compared to captive-bred birds. However, the origin of translocated birds (captive-bred or wild-caught) did not influence the survival during the initial 42 days of radio-tracking. The post-release survival of the birds was most critical during the initial week. A noteworthy secondary finding is that radio transmitters did not impact the birds' long-term survival, with survival rates remaining nearly identical between birds equipped with transmitters and those without.
Hard-released wild juveniles from the Inagua source population occupied an area 12–15 times larger than captive-bred juveniles released by soft release. This variation in home range may stem from several factors. Individuals bred in captivity, having become accustomed to regular human interaction and consistent provision of food and water, may possess limited experience in resource-seeking behaviours. Life within the confined space of indoor aviaries, characterized by limited environmental variation such as the absence of predators, simple habitat structures, and a lack of thermal sun-shade mosaic, might hinder these birds' exploration abilities in the more complex habitat into which they are released, especially given their early-stage development in such simplified conditions. In contrast, wild-caught birds, raised in natural environments, exhibit distinct advantages over their captive counterparts. They had experienced wider movement ranges, developed defense mechanisms against predators, and demonstrated proficiency in foraging within structurally complex habitats and under more varied thermal conditions. In addition, they were not dependent on humans for a steady supply of food and water. When released as solitary individuals in an unknown new area with younger pine forests, they lack social connections in an unrecognized home range. Their inherent mobility, experienced during their early life phases, may lead to larger movements in comparison to captive-bred birds. Volpe et al. (2014) have shown that movement behaviour of translocated birds reflects natural behaviour of un-manipulated birds. Thus, the differences in movement and home range size between captive-bred and wild blue chaffinches surely reflect the fact that the birds' behaviour during translocation is similar to their previous movement behaviour. A pivotal consideration in translocation is the stress experienced by both captive-bred and wild-caught birds. Captive‐bred birds may be less sensitive to capture and transportation than wild-caught birds (Jenni et al. 2015). Such stress can be exacerbated in wild-caught birds due to their unfamiliarity with human handling, possibly perceived as predatory threats, with consequences on their mobility and energy budget (Carrascal and Polo 1999). Measures like soft releases, where birds are kept in large cages for several days before being released, have been proposed as mitigation strategies to allow recovery from such induced stress. In line with this, our observations suggest that wild-caught birds, while benefiting from a broader range of experiences in their natural environments, may still be at a disadvantage in hard-release, particularly as solitary individuals in unfamiliar territories.
Dispersal over wide areas is often correlated with high mortality (Payevsky 2016; Dale 2001). This might disproportionately affect translocated wild birds as compared to captive-bred ones, especially during extensive post-release movements or when returning to their capture sites in the subsequent months. However, our data indicate that the mortality rate during the first 42 days post-release was consistent between the two groups, despite their distinct home range sizes. Moreover, mortality fell sharply within 7 days of release, showing that this is the most critical period in blue chaffinch translocations (see also Letty et al. 2000; Teixeira et al. 2007; Dickens et al. 2010). A possible compensatory dynamic might be at play: the extensive experience of wild-caught birds with natural habitats aids them in exploiting larger home ranges, which could offset the restricted mobility observed in captive-bred birds due to their unfamiliarity with the wild.
The main cause of mortality in the weeks following release was from natural predators (raptors), with normal mortality levels for juvenile birds (Cox et al. 2014). Although the sample size is too small to draw conclusions, mortality from raptors and collisions with cars on the road were exclusively associated with wild-caught juveniles from Inagua. This finding is consistent with the greater mobility and dispersal of wild-caught juveniles released one day after capture. In two cases, there was clear evidence of predation by a non-native species, feral cats. The negative impact of feral cats on wildlife, particularly in island environments, is well documented and widely recognised as a major concern for endangered species (Smucker et al. 2000; Woods and McDonald 2003; Medina and Nogales 2009; Medina et al. 2011; Palmas et al. 2017; Mori et al. 2019). Therefore, since this is an exotic species that preys on and threatens other endemic species (e.g., Gran Canaria Giant Lizard Gallotia stehlini, we recommend the implementation of a feral cat trapping programme in the current and potential distribution areas of the blue chaffinch, as well as in areas where future releases will take place (e.g., pine forest of Tamadaba).
There exists a notable caveat in our observational design: the interplay between wild-caught birds from the source population and captive-bred birds has not been effectively crossed with the hard-release versus soft-release strategies for translocated birds. This design constraint stems from a bird release strategy aimed at minimizing stress for wild birds, which were released a day after capture. Moreover, there was a pronounced concern regarding the potential loss of captive-bred birds. These birds, representing a significant financial and labor investment, are inherently more naive because of their controlled rearing environment. Utilizing a hard-release approach without prior acclimation to the release zone could jeopardize the substantial resources invested in their breeding and maintenance. Nevertheless, the contribution of wild translocated birds to the population was notably less than that of captive-bred individuals. A significant reason for this was the return of 18% of the wild translocated birds to their original location, preventing them from establishing connections between populations at La Cumbre. Delgado et al. (2020) highlight that very few captive-bred birds moved to Inagua pine forests (< 2%). Of the wild translocated birds that survived and returned to their original pine forests in Inagua from La Cumbre, less than 5% were subsequently observed again in La Cumbre. As a result, they did not contribute to the new population nor foster connections between the two populations. For future actions, we recommend implementing measures to better establish wild-caught birds at release sites. Utilizing soft releases can aid in their recovery from the stress of capture, handling, and transportation, while simultaneously offering protection from predators the first week after release (Beck et al. 1994; Letty et al. 2000; Wanless et al. 2002; Teixeira et al. 2007; Dickens et al. 2010). Mitchell et al. (2011) demonstrated the improved release success with soft-releases in species with a high tendency for dispersal or those likely to experience significant predation pressure. Such an approach also promotes a sense of familiarity with the new landscape and habitat they can see through the cage walls (Stamps and Swaisgood 2007) and encourages the development of social bonds with other birds in the same setting that can enhance future foraging activity while reducing the risk of predation (Carrascal and Moreno 1992; Lima 1995; Beauchamp 1998; Webber et al. 2023).
In conclusion, our research indicates that well-designed translocation programs can bolster or stabilize small populations of species at-risk, at least in the short term. Translocated birds exhibited impressive post-release survival rates and successfully established at the release site, supporting the population growth. Captive-bred birds contributed more significantly than wild-caught birds, primarily because a notable portion of the wild-caught birds returned to their original capture sites. Thus, the reinforced programme for the Gran Canaria Blue Chaffinch has been successful during its establishment phase. We advocate for continuous population monitoring of birds translocated to La Cumbre, putting emphasis on soft-release approaches to ensure the long-term efficacy of the programme.
References
Armstrong DP, Seddon PJ (2008) Directions in reintroduction biology. Trends Ecol Evol 23:20–25
Armstrong DP, Wittmer HU (2011) Incorporating Allee effects into reintroduction strategies. Ecol Res 26:687–695
Attum O, Cutshall C (2015) Movement of translocated turtles according to translocation method and habitat structure. Restor Ecol 23:588–594
Baíllo A, Chacón JE (2021) Statistical outline of animal home ranges: an application of set estimation. In: Handbook of statistics, vol 44. Elsevier, pp 3–37
Beauchamp G (1998) The effect of group size on mean food intake rate in birds. Biol Rev 73:449–472
Beck BB, Rapaport LG, Stanley PMR, Wilson AC (1994) Reintroduction of captive-born animals. In: Olney PJS, Mace GM, Feistner ATC (eds) Creative conservation: interactive management of wild and captive animals. Chapman & Hall, London, pp 265–296
Bernardo CSS, Lloyd H, Olmos F, Cancian LF, Galetti M (2011) Using post-release monitoring data to optimize avian reintroduction programs: a 2-year case study from the Brazilian Atlantic Rainforest. Anim Conserv 14:676–686
BirdLife International (2021) Fringilla polatzeki. The IUCN Red List of Threatened Species 2021:e.T103822640A205711288. https://doi.org/10.2305/IUCN.UK.2021-3.RLTS.T103822640A205711288.en. Accessed on 23 May 2023
Bivand R, Lewin-Koh N (2023) _maptools: Tools for Handling Spatial Objects_. R package version 1.1-8. https://CRAN.R-project.org/package=maptools
Brockett B, Banks S, Neaves LE, Gordon IJ, Pierson JC, Manning AD (2022) Establishment, persistence and the importance of longitudinal monitoring in multi-source reintroductions. Anim Conserv 25:550–565
Burnham KP, Anderson DR (1998) Model selection and inference: a practical information-theoretic approach. Springer, New York
Cabezas S, Calvete C, Moreno S (2011) Survival of translocated wild rabbits: importance of habitat, physiological and immune condition. Anim Conserv 14:665–675
Calenge C, Fortmann-Roe CFS (2023) _adehabitatHR: Home Range Estimation_. R package version 0.4.21. https://CRAN.R-project.org/package=adehabitatHR
Carrascal LM, Moreno E (1992) Proximal costs and benefits of heterospecific social foraging in Great Tit Parus major. Can J Zool 70:1947–1952
Carrascal LM, Polo V (1999) Coal tits, Parus ater, lose weight in response to chases by predators. Anim Behav 58:281–285
Carrascal LM, Moreno AC, Delgado A, Suárez V, Trujillo D (2017) Habitat suitability—density relationship in an endangered woodland species: the case of the Blue Chaffinch (Fringilla polatzeki). PeerJ 5:e3771
Carrascal LM, Delgado A, Suarez V, Moreno AC (2022) Population size, abundance, habitat relationships and the result of a translocation programme in the Gran Canaria Blue Chaffinch Fringilla polatzeki. Bird Conserv Int 32:460–475
Chichorro F, Juslén A, Cardoso P (2019) A review of the relation between species traits and extinction risk. Biol Conserv 237:220–229
Couet J, Marjakangas EL, Santangeli A, Kålås JA, Lindström Å, Lehikoinen A (2022) Short-lived species move uphill faster under climate change. Oecologia 198:877–888
Cox WA, Thompson FR III, Cox AS, Faaborg J (2014) Post-fledging survival in passerine birds and the value of post-fledging studies to conservation. J Wildl Manag 78:183–193
Dale S (2001) Female-biased dispersal, low female recruitment, unpaired males, and the extinction of small and isolated bird populations. Oikos 92:344–356
De Milliano J, Di Stefano J, Courtney P, Temple-Smith P, Coulson G (2016) Soft-release versus hard-release for reintroduction of an endangered species: an experimental comparison using eastern barred bandicoots (Perameles gunnii). Wildl Res 43:1–12
Delgado A, Calabuig P, Suárez V, Trujillo D, Suárez-Rancel MM (2016) Preliminary assessment of the release of captive-bred Gran Canaria Blue Chaffinches Fringilla teydea polatzeki as a reinforcement population. Bird Study 63:554–558
Delgado A, Suárez V, Trujillo D, Carrascal LM (2020) Dispersal and movements of the endemic Gran Canaria blue chaffinch (Fringilla polatzeki) between the source population of Inagua and the traslocation area of La Cumbre (Gran Canaria, Canary Islands). Technical Report LIFE14 NAT/ES/000077. https://doi.org/10.13140/RG.2.2.21665.433
Díaz L, Delgado A (2021) Un programa de translocación basado en la cría en cautividad y liberaciones logra el establecimiento de nuevas poblaciones en estado silvestre del pinzón azul de Gran Canaria (Fringilla polatzeki). Aviornis Internacional 176:20–29
Dickens MJ, Delehanty DJ, Romero LM (2010) Stress: an inevitable component of animal translocation. Biol Conserv 143:1329–1341
Duncan RP, Blackburn TM (2007) Causes of extinction in island birds. Anim Conserv 10:149–150
Fieberg J (2007) Kernel density estimators of home range: smoothing and the autocorrelation red herring. Ecology 88:1059–1066
Frankham R (1998) Inbreeding and extinction: island populations. Conserv Biol 12:665–675
Garcia-del-Rey E, Marthinsen G, Calabuig P, Estévez L, Johannessen LE, Johnsen A, Lifjeld JT (2013) Reduced genetic diversity and sperm motility in the endangered Gran Canaria Blue Chaffinch Fringilla teydea polatzeki. J Ornithol 154:761–768
Griffith B, Scott JM, Carpenter JW, Reed C (1990) Translocations of captive-reared terrestrial vertebrates 1973–1986. Endanger Species Update 8:10–14
Hardman B, Moro D (2006) Optimising reintroduction success by delayed dispersal: is the release protocol important for hare-wallabies? Biol Conserv 128:403–411
Illera JC, Delgado A, Trujillo D, Carrascal LM (2023) A multiple trait assessment provides insights into the short-time viability of a newly founded population in an endangered island passerine. Anim Conserv. https://doi.org/10.1111/acv.12927
IUCN (2013) Guidelines for reintroductions and other conservation translocations. Version 1.0. Species Survival Commission
Jenni L, Keller N, Almasi B, Duplain J, Homberger B, Lanz M, Korner-Nievergelt F, Schaub M, Jenni-Eiermann S (2015) Transport and release procedures in reintroduction programs: stress and survival in grey partridges. Anim Conserv 18:62–72
Karl BJ, Clout MN (1987) An improved radio transmitter harness with a weak link to prevent snagging. J Field Ornithol 58:73–77
Kays R, Crofoot MC, Jetz W, Wikelski M (2015) Terrestrial animal tracking as an eye on life and planet. Science 348:aa2478
Kemp L, Norbury G, Groenewegen R, Comer S (2015) The roles of trials and experiments in fauna reintroduction programs. In: Advances in reintroduction biology of Australian and New Zealand fauna, pp 73–90
Kraus B, McCallen E, Williams R (2017) Evaluating the survival of translocated adult and captive-reared, Juvenile Eastern Hellbenders (Cryptobranchus alleganiensis alleganiensis). Herpetologica 73:271–276
Letty J, Marchandeau S, Clobert J, Aubinearu J (2000) Improving translocation success: an experimental study of anti-stress treatment and release method for wild rabbits. Anim Conserv 3:211–219
LIFE+PINZÓN (2019) Project to increase range and Population size of the priority species Gran Canaria Blue Chaffinch, Fringilla polatzeki. Layman Report of LIFE+PINZÓN, LIFE14 NAT/ES/000077, p 20. http://lifepinzon.org/docs/Informe_Layman_ENG_Pinzon_Azul.pdf
Lifjeld JT, Anmarkrud JA, Calabuig P, Cooper JE, Johannessen LE, Johnsen A, Garcia-del-Rey E (2016) Species-level divergences in multiple functional traits between the two endemic subspecies of Blue Chaffinches Fringilla teydea in Canary Islands. BMC Zool 1:1–19
Lima SL (1995) Back to the basics of anti-predatory vigilance: the group-size effect. Anim Behav 49:11–20
Lucas PM, Gonzalez-Suarez M, Revilla E (2019) Range area matters, and so does spatial configuration: predicting conservation status in vertebrates. Ecography 42:1103–1114
Manes S, Costello MJ, Beckett H, Debnath A, Devenish-Nelson E, Grey KA, Vale MM (2021) Endemism increases species’ climate change risk in areas of global biodiversity importance. Biol Conserv 257:109070
McGowan PJ, Traylor-Holzer K, Leus K (2017) IUCN guidelines for determining when and how ex situ management should be used in species conservation. Conserv Lett 10:361–366
Medina FM, Nogales M (2009) A review on the impacts of feral cats (Felis silvestris catus) in the Canary Islands: implications for the conservation of its endangered fauna. Biodivers Conserv 18:829–846
Medina FM, Bonnaud E, Vidal E, Tershy BR, Zavaleta ES, Donlan CJ, Keitt BS, Le Corre M, Horwath SV, Nogales M (2011) A global review of the impacts of invasive cats on island endangered vertebrates. Glob Change Biol 17:3503–3510
Mitchell A, Wellicome T, Brodie D, Cheng K (2011) Captive-reared burrowing owls show higher site-affinity, survival, and reproductive performance when reintroduced using a soft-release. Biol Conserv 144:1382–1391
Mori EM, Menchetti A, Camporesi L, Cavigioli K, Tabarelli de F, Girardello M (2019) License to kill? Domestic cats affect a wide range of native fauna in a highly biodiverse Mediterranean country. Front Ecol Evolut 7:477. https://doi.org/10.3389/fevo.2019.00477
Palmas P, Jourdan H, Rigault F, Debar L, De Meringo H, Bourguet E, Vidal E (2017) Feral cats threaten the outstanding endemic fauna of the New Caledonia biodiversity hotspot. Biol Conserv 214:250–259
Payevsky VA (2016) Sex-biased survival and philopatry in birds: do they interact? Biol Bull 43:804–818
Pebesma E, Bivand R (2005) Classes and methods for spatial data in R. R News 5:9–13. https://CRAN.R-project.org/doc/Rnews/
R Core Team (2022) R: a language and environment for statistical computing. R Foundation for Statistical Computing, Vienna. https://www.R-project.org/
Sangster G, Rodríguez-Godoy F, Roselaar CS, Robb MS, Luksenburg JA (2016) Integrative taxonomy reveals Europe’s rarest songbird species, the Gran Canaria blue chaffinch Fringilla polatzeki. J Avian Biol 47:159–166
Scott JM, Carpenter JW (1987) Release of captive-reared or translocated endangered birds: what do we need to know? Auk 104:544–545
Seaman DE, Powell RA (1996) An evaluation of the accuracy of kernel density estimators for home range analysis. Ecology 77:2075–2085
Seddon PJ (2010) From reintroduction to assisted colonization: moving along the conservation translocation spectrum. Restor Ecol 18:796–802
Seddon PJ, Griffiths CJ, Soorae PS, Armstrong DP (2014) Reversing defaunation: restoring species in a changing world. Science 345:406–412
Smucker TD, Lindsey GD, Mosher SM (2000) Home range and diet of feral cats in Hawaii forests. Pac Conserv Biol 6:229–237
Stamps JA, Swaisgood RR (2007) Someplace like home: experience, habitat selection and conservation biology. Appl Anim Behav Sci 102:392–409
Staude IR, Navarro LM, Pereira HM (2020) Range size predicts the risk of local extinction from habitat loss. Glob Ecol Biogeogr 29:16–25
Taylor P, Crewe T, Mackenzie S, Lepage D, Aubry Y, Crysler Z, Woodworth B (2017) The Motus Wildlife Tracking System: a collaborative research network to enhance the understanding of wildlife movement. Avian Conserv Ecol 12:8
Teixeira CP, Schetini De Azevedo C, Mendl M, Cipreste CF, Young RJ (2007) Revisiting translocation and reintroduction programmes: the importance of considering stress. Anim Behav 73:1–13
Volpe N, Hadley A, Robinson W, Betts M (2014) Functional connectivity experiments reflect routine movement behavior of a tropical hummingbird species. Ecol Appl 24:2122–2131
Walther GR (2010) Community and ecosystem responses to recent climate change. Philos Trans R Soc B Biol Sci 365:2019–2024
Walther GR, Post E, Convey P, Menzel A, Parmesan C, Beebee TJC, Fromentin JM, Hoegh-Guldberg O, Bairlein F (2002) Ecological responses to recent climate change. Nature 416:389–395
Wanless R, Cunningham J, Hockey P, Wanless J, White R, Wiseman R (2002) The success of a soft-release reintroduction of the flightless Aldabra rail (Dryolimnas (cuvieri) aldabranus) on Aldabra Atoll, Seychelles. Biol Conserv 107:203–210
Webber QMR, Albery GF, Farine DR, Pinter-Wollman N, Sharma N, Spiegel O, Vander Wal E, Manlove K (2023) Behavioural ecology at the spatial-social interface. Biol Rev Camb Philos Soc 98:868–886
White GC, Burnham KP (1999) Program MARK: survival estimation from populations of marked animals. Bird Study 46:S120–S139
Whitford M, Klimley AP (2019) An overview of behavioral, physiological, and environmental sensors used in animal biotelemetry and biologging studies. Anim Biotelem 7:1–24
Wilson AC, Price MRS (1994) Reintroduction as a reason for captive breeding. In: Olney PJS, Mace GM, Feistner ATC (eds) Creative conservation. Springer, Dordrecht, pp 243–264
Woods MRA, McDonald SH (2003) Predation of wildlife by domestic cats Felis catus in Great Britain. Mammal Rev 33:174–188
Acknowledgements
The work was funded by the Cabildo de Gran Canaria and the LIFE 14 NAT/ES000077, Project of range expansion, and population size of the priority species Fringilla teydea polatzeki. The Cabildo de Gran Canaria carried out the reinforced programme and manages the Captive Breeding Centre. The Government of the Canary Islands authorised the release and marking of birds. We are grateful to two anonymous referees for their constructive comments that helped to improve the first version of the manuscript. Claire Jasinski kindly improved the English of several versions of the paper.
Funding
Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature.
Author information
Authors and Affiliations
Contributions
Conceptualization: AD; data curation: VS, DG, DT, AD; formal analyses: AD, LMC; writing: AD, LMC; review and editing: VS, DG, DT.
Corresponding author
Additional information
Communicated by F. Bairlein.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Supplementary Information
Below is the link to the electronic supplementary material.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Delgado, A., Suárez, V., González, D. et al. Establishment success of the reinforced population of Gran Canaria Blue Chaffinch Fringilla polatzeki. J Ornithol (2024). https://doi.org/10.1007/s10336-024-02172-1
Received:
Revised:
Accepted:
Published:
DOI: https://doi.org/10.1007/s10336-024-02172-1