Dynamics in spatial use by Ospreys (Pandion haliaetus) during the breeding season revealed by GPS tracking

The migratory behaviour of Ospreys (Pandion haliaetus) has been well studied using satellite telemetry, but studies on their behaviour in the breeding area are scarce. We caught and fitted 17 adult Ospreys with solar-powered GPS-enhanced satellite transmitters in northeast Germany and tracked them for up to 7 years. There was a high variation in home range size depending on sex, nesting site and breeding success. The home ranges of successfully breeding males (median 33.4 km2, 95% kernel density estimations) were significantly larger than those of females (median 4.6 km2) and varied in extent and shape between individuals. Some females made long excursions and sometimes spent the night at great distances to their nests after the juveniles had fledged but they also returned to the nesting area before they left for migration. The males’ home range areas were consistent during the breeding period. The proportion of water surfaces on the home range areas of males ranged between 9.6 and 29%. The overnight roosts were found in distances up to 7.83 km (median 980 m) for successfully breeding males and up to 105 km (median 568 m) for successfully breeding females. The correlation between overnight roost-nest distance and the corresponding home range area was significant for females but not for males. The overlap in the home ranges of individual males that occupied the same nests in different years varied between 37.3 and 54.7% of the mutual home ranges. Males partly included the same waterbodies in their home ranges, but they also visited different lakes. In the incubation and nestling phases, females are closely bound to the nest, whereas their mates can range over vast areas.


Introduction
Home ranges reflect the spatial use of habitats by individual animals performing their normal activities, such as foraging and reproduction (Burt 1943). Hence, they consist of places where animals' lives happen, i.e., where they mate, breed, eat and sleep. As a result of interactions between an individual's energy requirements and ecological or environmental factors, the sizes of home ranges vary (Newton 2010a). For example, the utilization of GPS telemetry revealed that in Red Kites (Milvus milvus) the size of home ranges is an indicator of habitat quality, which affects the rate of offspring (Pfeiffer and Meyburg 2015). Comprehensive data on the home ranges of animals that are highly mobile and, thus, can cover large distances in a short time, such as birds, are difficult to obtain. The technical progress and the development of transmitters that allow for following individuals by satellite tracking, however, enable researchers to uncover, for example, the migratory routes, movement patterns at breeding and wintering grounds, and home ranges of different bird species (e.g., Newton 2010b; Bairlein 2022). In recent decades, raptors have been subjected to telemetry studies not only due to their body sizes, which allow heavier and thus more efficient transmitters to be fitted, but also due to their lifestyle, as many raptors soar over open landscapes during the daytime, thus facilitating the use of solar-powered transmitters by prolonging their operating time (e.g., Strandberg et al. 2009;Moss et al. 2014;Vansteelant et al. 2015). One of these raptor species is the Osprey (Pandion haliaetus).
The Osprey has a virtually worldwide distribution and is one of the most studied bird species (Poole 2019). It inhabits a wide variety of habitats, and its migratory behaviour has been extensively studied using satellite telemetry (e.g., Martell et al. 2001Martell et al. , 2014Alerstam et al. 2006;Bedrosian et al. 2015;Monti et al. 2018;Anderwald et al. 2021;Meyburg and Holte 2023). Surprisingly, there are virtually no detailed studies of its home ranges or habitat use in the breeding area using GPS satellite telemetry, although its lifestyle is ideal for the use of solar-powered transmitters, as it usually perches on top of trees and man-made structures, flies only in open air and its short contour feathers do not cover the solar panel on the transmitters.
In this study, we analysed the ecology and behaviour, especially the dynamics in habitat use and home range size during the breeding season of 17 adult Ospreys from northeast Germany fitted with GPS-enhanced transmitters. We investigated the variation in home range sizes between the different stages of breeding as well as between the sexes and hypothesised larger and more constant home range sizes in males than in females. We analysed the overlap in the home ranges of different males using the same nest in different years as well as the proportion of water surface areas within the home ranges. We determined the location of overnightroosts and expected a positive correlation between overnight roost-nest distance and home range size.

Study area and data collection
The core area of Osprey breeding occurrence in Germany is located in the northeast in the federal states of Mecklenburg-Vorpommern and Brandenburg (Moll 1969;Meyburg et al. 1996;Herrmann 2018). The study was carried out in the vicinity of Lake Müritz (Fig. 1), the largest lake in northern Germany (112.6 km 2 ), where breeding success has been monitored for decades (Moll 1969;Meyburg et al. 1995;Herrmann 2018). The vast majority of Ospreys have been nesting on power pylons for many years, and few have been nesting in trees. We had the rare case of a female that went from being a pylon breeder to being a tree breeder.
In the period from 1995 to 2011, 28 adult Ospreys were marked with satellite transmitters (PTTs or Platform Transmitter Terminals) at 11 breeding sites; since 2006, 17 with solar-powered GPS-enhanced tags (Solar Argos/GPS PTTs; Microwave Telemetry, Columbia, MD, USA). Due to their higher accuracy, only the GPS-enhanced PTTs were considered in the statistical analyses.
The transmitters were deployed using a Teflon harness to fix the tag near the centre of the lift to minimise the effect on bird behaviour. The total weight of the PTT and harness was < 2.2% of the body weight of all Ospreys, which is lower than the recommended limit of 3% (Kenward 2000;Barron et al. 2010;Hebblewhite and Haydon 2010). GPS data, geographical coordinates as well as speed, bearing and altitude were collected every hour between 02:00 and 23:00 UTC by the PTTs whenever the batteries were sufficiently charged.
The Ospreys were caught using a dho-gaza trap near their respective nests using a live, adult White-tailed Eagle (Haliaeetus albicilla) as a decoy (Bloom 1987;Zuberogoitia et al. 2008). The Ospreys were captured during the last phase of the nestling period, as this was the time when the risk was lowest that the adults would abandon the nest, which did not occur. At that, the females seemed to be very tolerant of the human disturbance and handling, and, after release, directly returned to the nest in many cases. The individual Ospreys were tracked for up to 7 years (Table S1). As far as possible, the nestlings were ringed, so that almost half of the adults captured and tagged with transmitters were already ring bearers.

Spatial and statistical analyses
The statistical and spatial analyses were performed using R software (v4.0.3; R Core Team 2020). The maps were drawn using the 'OpenStreetMap' package (Fellows 2019).
GPS data of each individual were analysed for the respective time spent annually in the breeding area. Home ranges were calculated using 50% and 95% kernel density estimations (KDE50/KDE95) as well as 50% and 95% minimum convex polygons (MCP50/MCP95). Home range areas and distances were calculated in the UTM 33N reference system (epsg:25833). A Stamen Toner raster map (zoom level = 13) of the study area was used to identify water bodies and to calculate the respective areas of water surfaces within the home ranges of male Ospreys.
The home range areas of successful males and females were compared using the non-parametric Mann-Whitney U test. Additionally, KDE95 was performed per month (April to August) for each individual separately to investigate the dynamics of home ranges between different stages of breeding. We fitted two generalised additive mixed Fig. 1 Study site in northeast Germany. Letters: nests, red lines: home ranges (MCP95) of male Ospreys models (GAMMs) on home range size (KDE95) for males and females, respectively, including the individuals' IDs as random factors. Month was taken as an independent variable in the GAMMs, and a smoothing function was applied. To reduce the strong effects of outliers, the home range sizes were square root-transformed before fitting the models.
Overnight roosts of successfully breeding males and females were identified to investigate possible dynamics in distances to the nest from April to August. A location was considered an overnight roost when the first data point in the morning was located within a zone with a 100 m radius around the last data point in the previous evening and if on both occasions the bird was not in flight (speed = 0). The distances between these overnight roosts and the respective nest were calculated as great-circle distances and tested in dependence of the breeding stage (represented by months) using GAMMs. Because the transmitters were solar-powered, the charging level of the batteries varied during daytime, and single fixes were sometimes missing, due to e.g. cloudy conditions, especially at early morning and late evening hours when solar radiation was limited. Due to this variation, the fixes that are required for detecting an overnightroost were not available in several cases. For example, the last scheduled fix in the evening was at 23:00 UTC, but due to the low battery the last recorded fix was at 21:00 UTC and the bird has not been perched at the roost yet, or vice-versa in the morning when the bird was already active before the first fix could be recorded. For those nights, the detection of the overnight roosts was not achievable.
The number of nights for which a roost could be detected differed considerably among individuals and years (males: 2-52 nights with detected roosts; females: 8-131 nights). Hence, the frequency of use of a single roost could not be analysed and each overnight roost was considered only once per month regardless of the number of nights a bird spent there. The mean roost-nest distances of an individual bird per month and the corresponding home range area were tested for correlation (Pearson's product-moment correlation) for males and females separately.
At breeding sites where at least two tracked males used the same nest in different years, the home ranges (KDE95) of those males were compared by determining the overlap in home ranges. The area of overlap was computed as the proportion of the mutual area of both home ranges. In cases where data from one male were available for more than 1 year, only the home range of the first tracked year was considered in the analysis of overlap to keep the data comparable.
Long-distance excursions of successfully breeding females and the behaviour of a pair without breeding success were illustrated by means of home ranges computed as KDE50 on a daily basis.

Results
All the investigated females wintered in Africa, as did most of the males except for two that wintered in Iberia (Table S1). The home range areas (KDE95) ranged from 0.42 to 212.5 km 2 (excluding two females that made longdistance excursions, including them: 0.42 to 847.26 km 2 ) in successful breeders (Table 1). The home ranges of the two females without breeding success (ID81339: 6232 km 2 ; ID74991: 5893 km 2 ) exceeded those of the successful females (Table 1).
The proportion of water surfaces on the home range areas of males ranged between 9.6 and 29%, with the highest proportions found for nest D (Table 1, Fig. 3b). Male ID95783 was followed for 7 breeding seasons. In 2011, when the transmitter was fitted, the male bred in nest J and from 2012 to 2017 in nest L (1.96 km apart). The MCP50 home range in nest J (2011) was 4.6 times as large as the mean MCP50 home range in nest L (2012-2017), whereas the ratio of KDE95 home ranges was 1.6:1 (Fig. 4a, Table 1).
Male ID70107 was followed for 3 breeding seasons, always breeding in nest K in close proximity to Lake Müritz (Fig. 1). There are several fish ponds ~ 2 km south of the nest that were visited frequently by male ID70107 (e.g., in 2012; Fig. 4b). The same ponds were visited by male ID06982 from nest G (~ 14 km) in 2006. This bird did not visit Lake Müritz at all, which in turn was visited by male ID70107 only in the periphery (~ 100 to 200 m from the shore, max ~ 800 m).
The GAMM on home range dynamics of successful females revealed a non-linear relationship between square root-transformed home range area and month with the smallest home ranges in May and June (Tables S2 and S3, Fig. 5, Supporting information). For males, the GAMM showed no significant effects.
The maximum number of nights spent on one single overnight roost was found to be 42 for males (ID52029 in 2011) and 93 for females (ID68562 in 2013 and 2014). The maximum number of different roosts per individual was eight in males (ID95783 in 2017) and eleven in females (ID81339 in 2010). The overnight roosts were found to be in and directly next to the nest and in distances up to 7.83 km (ID94755; August) (median 980 m) for males and up to 104 km (ID74991; August) (median 568 m) for females (Supporting information). The GAMMs revealed a non-linear relationship between month and distance for both males and females (Tables S4 and S5, Fig. 6).
Regarding nests that were occupied by at least two different males in different years, data from two out of the eight breeding locations were available: nest D (occupied by males ID81340 in 2008, ID52029 in 2011 and ID94752 in 2009) and nest E (occupied by males ID74992 in 2007 and ID94755 in 2009). The overlap in the respective home ranges varied between 37.3 and 54.7% of the mutual home ranges (Table S6), and the males partly included the same waterbodies in their home ranges, but they also visited different lakes (Fig. 7).

Long-distance excursions of females
Females ID74991 (in 2008 and 2009) and ID94757 (in 2009 and 2010) made long excursions in late July or August and sometimes spent the night at a great distance from the nest (Fig. 8). Female ID74991 visited strange nests 111 km south of its own in both years. This location was visited four times in 2008, sometimes with several days between the excursions, and the bird stayed there for five nights in total. It always returned to the breeding area and departed for autumn migration from its nesting area (Fig. 8). Female ID94757 visited a location 63 km southwest of its nest and returned to the nest area the next day in both years. In 2009 the female departed for autumn migration from its nesting area, while in 2010, it flew to the remote location eleven days after the return from the first excursion, stayed there for two weeks and started migrating from this remote location (Fig. 8).

Behaviour of Ospreys without breeding success
In 2007 male ID74990 and female ID74991 were fitted with GPS PTTs as the only tracked pair in this data set. The two young were found dead in the nest on 26 June. Based on the behaviour of the parents, the chicks were assumed to be dead since 19 June because the female was located for the first time about 18 km from the nest but predominantly stayed in the breeding area at first (Fig. 9). On 23 June, the female went for another excursion for 22 days in approximately 20 km distance to the nest with two additional trips 55 km in north-eastern directions. From 16 to 30 July, the female was almost exclusively located close to the nest. From 31 July to 3 August it went on a long-distance excursion about 111 km south of its nest to the lakes west of the city of Potsdam,   (Fig. 9). On 13 August, the female departed from the nesting area for autumn migration, with another two-days stopover at the lakes west of Potsdam, thus not earlier than other females with breeding success. In total, the bird visited this area west of Potsdam 11 times during the tracking period.
In contrast, the male remained in the nesting area with maximal distances of 18 km until the start of outward migration on the morning of 15 September (Fig. 9), thus stayed as long as other successfully breeding males.

Breeding success of Ospreys with satellite transmitters
All Ospreys fitted with transmitters bred successfully throughout the study period, except of three birds. Male ID74990 together with female ID74991 lost their chicks in the early nestling stage in 2007 due to unknown circumstances. Female ID81339 did not breed at all in one out of four years (2009). It is the only case of a non-breeder in this dataset breeding successfully in other years. The records of these birds have not been considered in the home range analysis (breeding success and other topics will be discussed in detail in another paper; the overall good breeding success is taken as an indication that the tags caused only minor disadvantage (e.g., Hebblewhite and Haydon 2010;Therrien et al. 2012).

Discussion
This study reveals detailed dynamics and differences in spatial habitat use by adult male and female Ospreys during the breeding season as well as other results on biology in the breeding area, e.g., the length of stay at the end of the breeding season and the beginning of outward migration. GPS telemetry results on the spatial behaviour of non-breeding, unsuccessfully breeding adults and adult floaters are not yet available.
Overall, the by far largest home ranges were found for two females, one of which lost its brood in 2007 (ID74991) and the other (ID81339) did not breed at all in one out of four years, as well as two females with fledged young during the last few weeks before leaving for outward migration (ID74991 in 2008 andID94757 in 2009 and. These birds left for long distances during or at the end of the breeding season, respectively, while the successfully breeding individuals in most cases were more bound to the nesting area, which is reflected by the smaller home range sizes of the successful breeders in this study.
To our knowledge, there are no earlier descriptions of such behaviour of female Ospreys with fledglings which undertook long-distance excursions of up to over 100 km from the breeding site and returned during the breeding season. Such behaviour was first detected in individual adult female Lesser Spotted Eagles (Clanga pomarina) which move far away from the nest even before the young have fledged, thus risking the loss of the offspring, e.g., due to predation by Goshawks (Accipiter gentilis). The fact that they visit foreign nests directly and also stay there longer has been established by GPS telemetry, direct observations and DNA results from moult feathers (Meyburg et al. 2007(Meyburg et al. , 2022. Home ranges may also differ between populations and habitats. For instance, Bedrosian et al. (2015) reported home range size estimates (MCP95) of ~ 26.2 ha in adult males and ~ 346 ha in adult females breeding in northwest Wyoming (USA). This is 0.5% and 2.5% of the home range sizes found in our study (MCP95: males ~ 4864 ha, females ~ 13,577 ha, Table 1). The reasons are probably the breeding habitat, population density and location of the nest. In productive habitats with high abundances of fish, less pressure by conspecifics and a nesting location close to the hunting area, home ranges are likely to be small. This is also supported by our findings on home range sizes at nest K, which is located close to the Boecker fish ponds southeast of Lake Müritz. These fish ponds turned out to be more attractive to male ID70107 (which showed the smallest home range sizes among male Ospreys in this study) than the large Lake Müritz (Fig. 5b, Table 1). Male ID06982, which had the largest home range, often came from its nest G, ~ 14 km away, to forage at the Boecker fish ponds, but did not fly the short distance further towards the Müritz.
The reported difference in home range sizes between males and females is a result of the division of labour between the sexes during the breeding season. While females spend most of the time on the nest, first incubating the eggs and later caring for the nestlings, males only play a minor role (with huge individual variation) in incubating but provide the family with food (e.g., Siewert 1941;Moll 1962;Clancy 2006;Dennis 2007;Poole 2019).
The home ranges of female Ospreys are dynamic across the breeding season: in April when the birds have arrived and pairs are established, female Ospreys spend a great amount of time at the nest to defend it against intruders as well as for building and repairing purposes (Poole 2019), and for egg laying; in May and June, the females are closely bound to the nest while they incubate the eggs, care for the young, and defend the nest against predators, which leads to minimal home ranges during this phase; with increasing age of the young, the females make longer trips and gradually enlarge their home ranges until they leave the family for pre-migratory movements or migration soon after the juveniles are fully fledged (Bustamante 1995;Poole 2019). In particular, the two females ID74991 and ID94757 had bred successfully and made long excursions after the juveniles had fledged. ID74991 visited other nests 111 km apart at different days in July and August in two subsequent years (Fig. 8). Both birds returned to the nesting area. Similar movements during the breeding season have been reported in one other species, the Lesser Spotted Eagle (Meyburg et al. 2007(Meyburg et al. , 2022, but not yet in Ospreys. Simultaneously, the males hunt for fish to feed themselves and to supply the females and the young with food, which is reflected in more consistent home range areas throughout the months, with home range sizes of approximately 4-200 km 2 (KDE95).
The distances between the overnight roosts of females and the nest followed the same dynamics as their home ranges. The nearest roosts were detected in May and June, and the distances increased starting in July. The distances of the roosts of females increased more than those of males black squares: location of the nest, green triangles: last GPS location before migration, red lines: movements between GPS locations. Lower panels: distances to the nest. Grey/white bars separate days. Triangles: nights that have been spent on excursion far from the nest because the females gradually abandoned their parental duties and partly made long trips after the young had fledged before they departed for migration. The roost-nest distances of 63 and 104 km correspond to those two females that made the largest excursions in July and August.
During June, when the young were growing fast and required large amounts of food, the overnight roosts of males were closest to the nest. With increasing independence of the juveniles, during late July and August, the males increased the distances. The nests were still part of the males' home ranges, as the delivery of food to the fledged young is suggested to predominantly take place there (Bustamante 1995;Poole 2019), at least at the beginning of post-fledging dependant period, until the males and juveniles also started migration.
There is almost no information on floating behaviour for migratory species (Sergio et al. 2009). Almost no studies including those of the Osprey have been capable of Fig. 9 Behaviour of a breeding pair without success in 2007. Upper panel: distances to the nest of male ID74990 (dark grey) and female 74991 (light grey). Red vertical dashed line: day of the assumed loss of the young. Grey dashed lines: last observation in the breeding area before onset of the autumn migration. Lower panel: daily home ranges (KDE50) of female ID74991 in June (black), July (red), and August (yellow). Black square: nest monitoring the nonterritorial sector of a population because of the typically secretive behaviour of floating individuals (Kenward et al. 2000;Newton and Rothery 2001). None of the usual theories concerning floaters (Hunt 1998) seems to apply to the bird we tracked. Why the male ID16869 (Argos PTT) of unknown age stopped breeding and departed very late in spring from the African wintering grounds after having raised three young in the previous breeding season remains unknown.
Ospreys breed in different densities and in diverse habitats, on the coast, on islands, and at lakes and rivers. The few studies carried out thus far point to the need for further investigations using GPS telemetry to uncover additional details of Osprey ecology.