Introduction

Hybridization is a phenomenon that shapes the diversity of organisms, but is often underestimated in biological studies (Baack and Rieseberg 2007). Interspecific cross-breeding is relatively common in birds (Randler 2006), or perhaps this group is just better investigated because detection of hybrid individuals is easier, due to characteristic plumage and voices. Indeed, there are numerous studies that describe hybrids in various groups of birds. This propensity to hybridize probably contributes to the high rate of bird speciation and the presence of multiple species groups or species pairs of relatively young evolutionary age (Prager and Wilson 1975) as hybrids in birds are more frequent in sister than non-sister taxa (Gholamhosseini et al. 2013).

Hybrids have been reported quite often between pairs of woodpecker species and the majority of cases concern species from North and South America (e.g., Seneviratne et al. 2016). In Europe, hybrid individuals have been reported between the Green Picoides viridis and Grey-headed P. canus Woodpeckers (Ławicki et al. 2015) and between the Great-spotted Dendrocopos major (hereafter GW) and Syrian D. syriacus (hereafter SW) Woodpeckers (Gorman 1997; Dudzik and Polakowski 2011). The species in the latter pair are sibling taxa, as they have very similar phenotypes and genotypes, whereas they differ with respect to their ecologies (habitat choice) and ethology (territorial behaviour), but SW is much less known than GW (Pasinelli 2006). Moreover, SW has expanded into the range of GW in Europe (Michalczuk 2014), which has enabled and facilitated their cross-breeding. It is possible to identify GW × SW hybrids on the basis of their plumages; however, their characters vary substantially (Dudzik and Polakowski 2011). Furthermore, preliminary genetic studies have confirmed hybrid genotypes and indicated that hybrids are fertile as there were apparent backcrosses (Michalczuk et al. 2014). On the other hand, there have been no studies showing the structure of the sympatric populations with respect to the presence of mixed pairs and hybrids.

The aim of this study was to estimate the following: (1) what is the share of mixed pairs in urban populations and whether the cross-breeding is random with respect to sexes; and (2) how frequent are hybrid birds and what is the sex ratio of hybrids.

Materials and methods

The present study was based on woodpecker inventories executed during a project on SW and GW ecology and ethology in urban populations. Data were collected in 2013-2015 years in four urban areas (cities/towns) in Poland: (1) Radom, centre point 51.4024°N, 21.1650°E, (2) Grodzisk-Milanówek, c.p. 52.1145°N, 20.6614°E, (3) Krakow, c.p. 50.0205°N, 19.9963°E and (4) Wieliczka, c.p. 49.9813°N, 20.0685°E; in total, 80 km2). Details about the study areas, inventory techniques and data collection are presented elsewhere (Figarski 2014; Kajtoch and Figarski 2017). The total number of pairs of both species, mixed pairs (of [apparently] “pure” individuals or with hybrids, based on plumage characters) and hybrid individuals were counted and used to describe the population structure of these species in urban ecosystems. Next, we summarized all data about mixed pairs collected during inventories, obtained from observers in these cities and its vicinities, and data available in the work of Dudzik and Polakowski (2011). This database was used to estimate the frequency of sexes of particular species in mixed pairs. Finally, we used data about dead birds delivered since 2013 to the Collection Unit of the Institute of Systematics and Evolution of Animals, Polish Academy of Sciences in Krakow (hereafter ISEA PAS). We considered only birds found dead in Krakow city, and we restricted the analysis to juveniles, which prevailed among dead woodpeckers. We considered hybrids to be individuals that possessed intermediate characters in their plumage (see details specified in Table 1 and some exemplary photographs of hybrids shown in supplementary file 1).

Table 1 Basic characteristics that allow for differentiation of Syrian and Great-spotted Woodpeckers
Full size table

Results

In total, we identified 94 territories of woodpeckers and 192 individuals, including hybrids (regardless if paired or not). We found that for each “pure” pair of the GW, there was 0.7 pair of SW, but this ratio differed greatly between study areas (Table 2). Only in seven cases did conspecific territories (SW♂ × SW♀ with GW♂ × GW♀) overlap to some extent (7.4% of territories, Table 2). On average only 2.1% of territories (two pairs, Table 2) were formed by birds belonging to different species, but if also considering pairs comprising at least one hybrid, it was on average 5.3% (five pairs, Table 2). In total only seven individuals (3.6%, Table 2) could be identified as hybrids on the basis of their mixed plumage characters. Among these hybrids there were four females and three males [sex ratio (f/m) = 1.3], but if also considering hybrids listed in the work of Dudzik and Polakowski (2011) (four females and five males), the sex ratio (f/m) was 1.0.

Table 2 Basic characteristics of Syrian and Great-spotted Woodpecker populations in urban landscapes of Poland with emphasis on mixed pairs and hybrids occurrence
Full size table

Considering all available observations of mixed pairs (this study and some occasional observations, including data from Dudzik and Polakowski (2011)) (N = 14, including four pairs with hybrid members) with data available on the sex and species of individuals, 71.4% of pairs comprised a SW female and a GW (nine cases) or hybrid male (one case). Described in another way, 92.8% of pairs comprised a SW or hybrid female, and a GW (ten cases), SW (two cases) or hybrid male (one case).

Among dead juveniles collected from the city of Krakow, there were a total of 29 birds, 22 of which were assigned to GW, five to SW and two had intermediate characters (6.9% of birds were probable hybrids).

Discussion

Woodpeckers are a group of birds with relatively common interspecific cross-breeding (Gorman 1997; Ławicki et al. 2015). It seems that this phenomenon is especially pronounced at the border of two species’ ranges or when one species changes its range and starts to breed sympatrically with its congener. In such cases, the number of individuals of one or both species is low, and a lack of mates of the same species forces birds to mate and breed interspecifically. The latter case (range change) is especially interesting in the case of the SW and GW pair, as the former species is a newcomer in Central and Eastern Europe (Michalczuk 2014). Hybrids of both species have been reported across the SW range established in Central Europe since its expansion from the Middle East across the Balkans in the twentieth century (Michalczuk 2014). Notably, the number of hybrids seems to increase towards to the north, and in Poland such mixed birds are present more than just incidentally (Dudzik and Polakowski 2011). The presented note shows for the first time that mixed pairs of these woodpecker species (or hybrids) can constitute approximately 5.3% of woodpecker populations, at least in urban areas. The share of hybrid birds in these populations is more difficult to establish as probably only first generation hybrids can be easily identified in the field. Preliminary genetic studies have suggested that even approximately 20% of woodpeckers from sympatric populations could be of hybrid origin (Michalczuk et al. 2014). Examination of dead juveniles supports this suggestion as 7% of birds display intermediate plumage. It is also interesting that among hybrids there was an equal share of females and males. The question is if all hybrids originated from mixed pairs or if some hybrids resulted from extra-pair copulation, which is a phenomenon not observed in GW (Michalek and Winkler 2001) (there are no such studies for SW).

Finally, this study presents for the first time data that leads to the hypothesis that cross-breeding between SW and GW is not random, as almost all mixed pairs were found to comprise SW or hybrid females and GW males. This suggests the existence of some factors responsible for this sex-biased mating. Some differences in ethology of these species may make SW females more likely to mate with GW if there is a lack of unpaired SW males (Figarski 2017). It is also interesting that there were several examples of mixed pairs in which one bird was of hybrid phenotype, which suggests that D.syriacus × D.major offsprings should be viable (which is consistent with preliminary genetic data; Michalczuk et al. 2014). Also, the even sex ratio in hybrid individuals indicates that there should not be negative effects of interspecific cross-breeding.

This study brings new light on hybridization of SW and GW. Detailed population genetic analysis on numerous specimens would confirm or extend the knowledge on the extent of this phenomenon. These data could be important not only from an evolutionary perspective but also for the conservation of SW, which is protected in the European Union (annexed in the Birds Directive) and is still rare in some countries like Poland, Czechia and Austria. Hybridization could be one of the crucial factors, apart from habitat loss, responsible for halting expansion and further decline of the species in the northernmost part of its range.