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# Timing of departure from natal areas by golden eagles is not constrained by acquisition of flight skills

Arzhela Hemery, Lucas Mugnier-Lavorel, Christian Itty, Olivier Duriez,

Aurélien Besnard

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Arzhela Hemery, Lucas Mugnier-Lavorel, Christian Itty, Olivier Duriez, Aurélien Besnard. Timing of departure from natal areas by golden eagles is not constrained by acquisition of flight skills. Journal of Avian Biology, 2023, 2023 (7-8), pp.e03111. 10.1111/jav.03111. hal-04927686

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Research article

# Timing of departure from natal areas by golden eagles is not constrained by acquisition of flight skills

Arzhela Hemery<fnref n="1" />, Lucas Mugnier-Lavorel<fnref n="1" />, Christian Itty<fnref n="2" />, Olivier Duriez<fnref n="1" /> and Aurélien Besnard ID

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Correspondence: Aurélien Besnard (aurelien.besnard@cefe.cnrs.fr)

Journal of Avian Biology

2023: e03111

doi: 10.1111/jav.03111

Subject Editor:
Judy Shamoun-Baranes
Editor-in-Chief: Staffan Bensch
Accepted 07 March 2023

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The post-fledging dependence period (PFDP), which extends from a fledgling's first flight out of the nest to its departure from the parents' territory, is crucial in the life-cycle of birds. During this period, juveniles develop their flight and foraging skills to become fully independent. Despite the importance of this life stage in basic bird ecology and conservation, it remains largely overlooked – notably its link with the acquisition of flight skills. In this study, we modeled the variation in seven proxies describing flight skills of 84 GPS-tracked golden eagle juveniles in France between 2016 and 2020. Juveniles had a long but highly variable PFDP, averaging 177.9 (± 62.2) days after departure from the nest. This period is divided into two phases: a first phase of rapid increase in flight skills over the first 60 days after departure from the nest, followed by a plateau in which flight skills no longer develop until independence. These results suggest that the full development of flight skills is not a constraining factor during the PFDP and that it is advantageous for juveniles to choose to remain in their natal territory. We posit that parents' tolerance of fledged juveniles is a type of parental care that may maximize their own fitness by improving the survival of their descendants. In future studies, it may be of interest to investigate the factors that may explain the high variability in the duration of this stage between individuals within the same population.

Keywords: *Aquila chrysaetos*, dispersal, flight, GPS tracking, parent-offspring conflict, post-fledging dependence period, raptor

## Introduction

Following the nesting period and before starting the independence phase, juveniles of most altricial bird species exhibit a period of dependence on their parents, commonly referred to as the post-fledging dependence period (hereafter, PFDP) (Newton 1979, Sarasola et al. 2018). This period is defined as the time elapsed between the juvenile's first flight and its independence, i.e. until its departure from the natal territory (Newton 1979, Weston et al. 2013, Boileau and Bretagnolle 2014). During this period, parental

© 2023 The Authors. Journal of Avian Biology published by John Wiley & Sons Ltd on behalf of Nordic Society Oikos

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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www.avianbiology.org

<footnote n="1">¹CEFE, Univ. Montpellier, CNRS, EPHE-PSL Univ., IRD, Montpellier, France</footnote>

<footnote n="2">²Association BECOT (Banding and Research for Bird Conservation), Saint-Gervais sur Mare, France</footnote>
care can be direct, as when parents provide food or protection against predators (Ar and Yom Tov 1978), or indirect, if parents simply tolerate juveniles in their territory. Among birds, this period of parental care is highly variable between species (Naef-Daenzer and Grüebler 2016), ranging from ending when the fledgling leaves the nest, as for most seabird species such as the wandering albatross *Diomedea exulans* (Schreiber and Burger 2001, Fay et al. 2016), to a relatively long period after fledging, as for some passerines and most raptor species (Radford and Ridley 2006, Sarasola et al. 2018). It can also strongly vary between species within a given taxonomic group or even between individuals of the same species. For instance, the PFDP ranges from 3 to 31 days for the common kestrel *Falco tinnunculus* (Boileau and Bretagnolle 2014), while for the harpy eagle *Harpia harpyja*, this period lasts more than two years (Muñiz-Lopez et al. 2012, Urios et al. 2017). For species with a very long PFDP, it may encroach on the next breeding year and may be responsible for biennial breeding in some raptor species (Clark et al. 2020 for the Philippine eagle *Pithecophaga jefferyi* and in Kemp et al. 2020 for the martial eagle *Polemaetus bellicosus*).

Extending the care of juveniles after fledging represents a significant cost for the parents, which is balanced out by an increase in offspring survival (Naef-Daenzer and Grüebler 2016, López-Idiáquez et al. 2018). For adults, the optimal duration of the PFDP should therefore be the one which maximizes their fitness, i.e. both their own survival and that of their descendants. For species without cooperation between immatures and adults in raising the next offspring, the optimal strategy for juveniles should be to maximize their own fitness, sometimes apparently regardless of the impact on the parents' fitness (Ferrer 1992, López-Idiáquez et al. 2018). This conflict between parents and offspring leads to an evolutionary trade-off in the duration of the PFDP, which is influenced by several factors linked to juvenile, and potentially to the parents' states (Bustamante and Hiraldo 1989, Arroyo et al. 2002, Guo et al. 2010). Because of this trade-off, for several species the timing of independence coincides with the juvenile's acquisition of the ability to self-feed and move efficiently (Arroyo et al. 2002, Salinas-Melgoza and Renton 2007, Martens et al. 2018). Yet the decision of juveniles to leave the natal territory is also known to be closely related to the individual's body condition (Clobert et al. 2012, Boileau and Bretagnolle 2014) and to the reduction in parental investment in food supply (Bustamante 1993, Radford and Ridley 2006, Maness et al. 2011). For species with sexual size dimorphism, the length of the PFDF differs between sexes and is longer for the larger sex (Kenward et al. 1993, Weston et al. 2017).

Unlike the period of nest dependence or the dispersal phase, few studies have focused on the PFDP in birds, despite its crucial importance for population dynamics and therefore for species conservation (Delgado et al. 2009, Cox et al. 2014, Naef-Daenzer and Grüebler 2016). This stage is thus considered to be a knowledge gap in the understanding of the life history of many bird species (Cox et al. 2014, López-López et al. 2014, Naef-Daenzer and Grüebler

2016). Of studies that have focused on the PFDP, most deal with parental care behavior and its impact on juvenile survival (reviewed by Langen 2000, Cox et al. 2014, or Naef-Daenzer and Grüebler 2016). Its duration, and notably the relation between its duration and flight skill acquisition in juveniles, has been little studied because of technical limitations (Miller et al. 2019, Rymesova et al. 2021). Yet, first flights after departure from the nest do not mean that their flight skills are sufficient to be autonomous. Flying is a complex form of locomotion and a high level of flight control is required to hunt, move in the airspace or interact with others. Young birds need to practice because acquiring flight behaviour as efficient as an adult is fundamental for their survival (Ashmole and Tovar 1968, Yoda et al. 2004, Ruaux et al. 2020). It will allow them to move around and use food resources optimally (Bustamante and Hiraldo 1989, Sarasola et al. 2018, Corbeau et al. 2020). The few studies that have focused on this relationship observed that bird mobility gradually increased after fledging and that the acquisition of soaring flight seemed a critical prerequisite for the independence of juveniles (Bustamante and Hiraldo 1989, Ferrer 1992, Bustamante 1993, O'Toole et al. 1999). Yet Ferrer (1992) found that the acquisition of soaring flight appeared halfway through the PFDP of the Spanish imperial eagle *Aquila adalberti*, suggesting that this is not the only factor modulating the duration of this period. However, these studies remain very patchy, as they are mainly based on radio telemetry (VHF tracking) and visual field observation, methods with very limited spatial and temporal resolution (as mentioned in Cadahía et al. 2008).

More recently, a few studies using newer technologies such as GPS transmitters have been able to document the link between flight skills and the PFDP more accurately. For example, Weston et al. (2017) found that the PFDP of the golden eagle *Aquila chrysaetos* was divided into two phases: a first phase lasting the initial two months until juveniles reached full mobility ('ontogenetic phase') and a second phase in which juveniles remained in the adults' territory for several additional weeks ('maintained phase'). Yet that study used only the juvenile's distance from the nest with one location per day, a proxy that may be too rough to capture flight skills improvement. For another population of golden eagles in another geographical context (in Spain), Soutullo et al. (2006) did not find those two separate phases and they estimated the PFDP duration about 60-120 days after fledging. Ramos et al. (2019) identified the existence of many phases of exploration of juvenile Spanish imperial eagles outside the natal territory during the PFDP, suggesting that juveniles acquired their flying skills long before their definitive independence and supporting Ferrer's (1992) results. However, that study also relied on a single proxy, i.e. the total distance traveled by individuals at each exploration phase outside the natal territory. Further studies analyze more closely flight skills using several complementary proxies – such as in Corbeau et al. (2020) (cumulative distance, distance from the colony, ascension rate, etc.) in a study of great frigate birds *Fregata minor*.
In this study, we investigated the PFDP of 84 golden eagle juveniles, which were fitted with GPS transmitters at the nest in southern France to study the link between the acquisition of flight skills and the decision to leave the natal territory. We described the development of flight skills using seven proxies such as daily distance from the nest, daily average speed, daily flight time proportion, etc. We explored two alternative hypotheses. First, juveniles are constrained to remain in their natal territory because of limited flight ability. We predicted that flight skills would gradually improve throughout the PFDP until independence. If this prediction is true, then early emigrants would show flight skills that improve faster than late emigrants. We also predicted a slower improvement in flight skills and thus later independence for females, since due to the strong sexual dimorphism of this species, females may take longer to master certain flying techniques due to their larger size. Our alternative assumption, based on the results of Weston et al. (2017), was that juveniles stay in their natal territory even when they have full flight ability for other factors (such as a better chance to find food in winter hard conditions, etc.). For many raptor species, it takes months and even years for young birds to master flight and hunting skills similar as adults (Mundy et al. 1992, Harel et al. 2016, Davis et al. 2021). In this case, we predicted that PFDP would be decoupled into two phases: a first phase of improving flight skills, then a second phase (of varying length) during which flight skills no longer develop until independence decision. If this prediction is true, the flight skills acquisition rate would be independent of early/late independence state or sex.

# Material and methods

## Study species

The golden eagle is a large raptor that occurs in a wide range of habitats in the Holarctic (Cramp and Simmons 1980, Watson 2010, Katzner et al. 2020). This species hunts mainly medium-sized mammals and birds in open areas or scrub (Pedrini and Sergio 2002, Sergio et al. 2006, Watson 2010). It exhibits sexual dimorphism, with females having a wing length on average 10% larger than males and a body weight that averages 40–50% heavier (Watson 2010). They form monogamous pairs and settle and defend territories between 25 and 120 km², usually building their nests in cliffs or in tree canopies (McGrady et al. 2002, McLeod et al. 2002, Sergio et al. 2006, Watson 2010). Pairs usually have one or two fledglings each year (mean productivity is about 0.5 fledglings per year and is density-dependent, Chambert et al. 2020), which leave the natal territory between autumn and early spring (Pedrini and Sergio 2002, Soutullo et al. 2006, Weston et al. 2013). In France, the species is sedentary and breeds only in mountain ranges, namely the Alps, Massif Central, Pyrenees, and Corsica (Issa and Muller 2015).

## Study area

We collected data on juvenile golden eagles in three geographical areas in southern France (Supporting information). The Hautes-Alpes and Drôme study sites are both located in the Alps or the Alpine foothills, and have a large elevational variation (range 250–4102 m). The Hautes-Alpes have a typical mountain climate, with extended snow cover during six months in winter and cool summer temperatures, while the Drôme has a Mediterranean influence, with high temperatures in summer (25–30°C) and mild winters with limited snow cover (Joly et al. 2010). The Massif Central has low to medium elevations (75–1753 m), resulting in a more temperate climate with little snow during winter (Joly et al. 2010). However, some areas of the Massif Central are subject to high snow cover (areas above 1500 m) while others are subject to intense heatwaves in summer (the southernmost sites). In the Massif Central, golden eagle pairs have much larger territories (between 88 and 168 km²) than in the Alps (between 31 and 51 km²), due to the difference in topography and limited food resources (Issa and Muller 2015, Hemery and Itty unpubl.).

In France, a ringing program of golden eagles started in 2010. Since that year, 140 breeding events were monitored intensively from laying until juveniles left their nest. In this sample laying occurs around 20 March (range: 27/02–30/04'); hatching occurs around 1 May (range: 11/04–27/05); departure from the nest occurs around 20 July (range for males: 21/06–22/08 ; range for females: 06/07–13/08).

## GPS tracking data

Individuals were equipped with a GPS tag at the nest at approximately 48–58 days old (Supporting information). Age and sex were determined according to bird's size and plumage (Mathieu 1985). The sex was confirmed a posteriori using methods for molecular sexing with feathers and the pair of primers s 2550F – 2718R (Fridolfsson and Ellegren 1999). A total of 98 juvenile eagles were fitted with GPS/GSM tag between 2016 and 2020 (Personal Banding Program no. 579 authorized by the French banding center CRBPO). Only 84 juveniles (36 females and 48 males) from 54 natal territories were used for this study, as 14 juveniles died prematurely or had transmitter failures after a few weeks (Supporting information).

The tags were fitted using a 14 mm Teflon Bally Ribbon harness, using a backpack technique (Anderson et al. 2020) (Supporting information). Six models, from four manufacturers (Ornitela, Ecotone, E-obs and Microwave telemetry) have been used (Supporting information): all had a rechargeable battery via an integrated solar panel. The whole device weighed between 74 and 105 g: for all individuals, this was below the 3% of body weight suggested by Kenward (2001).

GPS tags were set to record 3D positions at intervals of between 5 and 15 min. In sunny weather, with a fully charged battery, the device could record at 1 min interval. Simultaneously to recording each position, GPS transmitters
systematically recorded the altitude (relative to ellipsoid or mean sea level, depending on the manufacturer of the device), the speed of the bird and the tri-axial accelerometry. Accelerometry was recorded during 5 s at a sampling rate of 50 Hz. In order to obtain the same reference altitude for all individuals, from sea level, all ellipsoid altitudes were converted to mean sea level altitudes (Péron et al. 2020). For birds fitted in 2019 and 2020 with Ornitela devices, GPS tags were set to record positions at 1 s interval as soon as the bird was in flight (defined as high-resolution data thereafter). Active motion was automatically detected by the accelerometer monitoring bird movement every second and analysed via an internal program designed by Ornitela in which the trigger threshold can be set at which the animal is considered to be in motion. Once active motion was detected, the transmitter automatically started logging GPS position and others sensor data every second. For stopping this intensive logging, the transmitter continuously analysed GPS speed and once several consecutive positions showed no movement, the transmitter went back to its standard GPS settings. The trigger threshold for this active motion can be modified by the users and we manually calibrated it specifically for golden eagles in 2019 (when this new option became available) by increasing or decreasing the threshold and then looking at the data recorded, whether all movements had been detected and recorded at high-resolution. For juveniles we began with a threshold value of ten to decrease gradually to seven (the threshold value used for adults). This was possible because we had some observational data for some of those eagles in 2019.

## Data preparation

The GPS data were stored on the www.movebank.org online platform, and then extracted and edited using R studio software ver. 2.5033 (www.r-project.org). We determined the date of the departure from the nest of each juvenile through careful visual examination of the data or by visits to the nesting sites when the battery logger was low (about twice a week, when the estimated date for departure from the nest was approaching). This step requires manual determination since the GPS signal is often inaccurate around the nest due

to interference with adjacent topography (the tree canopy or cliff). The datasets were also cleaned up to remove outliers, especially inaccurate locations. For this, we used the recommendations from Gupte et al. (2021) and did a spatio-temporal filtering by deleting impossible dates and locations.

We determined the PFDP duration using the Weston et al. (2013) method. These authors compared 12 different methods to find the best one to evaluate the dispersal's date for young golden eagles. This considers that independence corresponds to the day when the juvenile is recorded over 9 km from the natal nest and does not return to within 6 km of the natal nest for the following 10 days. We verified that the juveniles had really left their natal territory after the threshold by looking at the GPS data for each individual. We plotted the distribution of the PFDP durations and defined a threshold day, which is the median independence day, in order to split the individuals into two relatively balanced groups. The juveniles with a PFDP duration less than or equal to the threshold were categorized as 'early emigrants' while those with PFDP of more than the threshold were categorized as 'late emigrants'. We did a student t-test to check if there was a difference in PFDP duration between males and females. We tested the correlation between the PFDP duration and the date of departure from the nest with a Pearson correlation test.

To quantify the acquisition of flight skills, we estimated seven proxies resulting from the combination of location, altitude and speed data to characterize the movements of juveniles (Table 1). Those proxies reflect how far ('maximum distance from the nest' and 'cumulative daily distance'), how high ('maximum flight height' and 'average flight height'), how fast ('maximum speed' and 'average speed') and how long ('flight time proportion') juveniles fly day after day since the departure from the nest. This allows to evaluate the improvement in the distances covered, as well as the characteristics of their trips (at what speed, at what height) and thus to understand the evolution of skills over time. These proxies were calculated for each individual for each day. For each individual, we considered the firsts 300 days after departure from the nest to focus on the PFDP. To determine the flight height, we calculated the difference between the altitude

<table><caption>Table 1. The seven proxies used to characterize the development of flight skills of young golden eagles during the first 300 days after departure from the nest (calculated for each individual for each day).</caption><thead><tr><td>Proxies</td><td>Description</td><td>Unit</td><td>Number of individuals concerned</td></tr></thead><tbody><tr><td>Maximum distance from nest</td><td>Farthest Euclidean distance from the nest at which an individual has been located on a given day</td><td>Kilometer (km)</td><td>84</td></tr><tr><td>Cumulative daily distance</td><td>Cumulative Euclidean distance of all the successive locations on a given day</td><td>Kilometer (km)</td><td>84</td></tr><tr><td>Maximum flight height</td><td>Maximum flight altitude above the ground reached on a given day</td><td>Meter (m)</td><td>78</td></tr><tr><td>Average flight height</td><td>Average flight altitude above the ground for all locations on a given day</td><td>Meter (m)</td><td>78</td></tr><tr><td>Maximum speed</td><td>Maximum ground speed reached on a given day</td><td>Kilometer per hour (km/h)</td><td>78</td></tr><tr><td>Average speed</td><td>Average ground speed of all locations on a given day</td><td>Kilometer per hour (km/h)</td><td>78</td></tr><tr><td>Flight time proportion</td><td>Proportion of time spent in flight over all locations on a given day</td><td>Percentage (%)</td><td>78</td></tr></tbody></table>
provided by the GPS and the ground elevation derived from a Digital Elevation Model with a 25 m resolution provided by the French Natl Inst. of Geographical and Forest Information (IGN). However, because altitude and ground speed data are less reliable when the bird is perched compared to data acquired in movement (Shamoun-Baranes et al. 2012, Poessel et al. 2018, Péron et al. 2020, Acacio et al. 2022), we only retained data with speeds > 1 km h⁻¹ for altitude and speed proxies calculations. This threshold value was chosen according to aerodynamic theories, that this speed is too low to support an object in flight (Shamoun-Baranes and van Loon 2006, Pennycuick 2008). Even though there are several studies with a higher threshold of 3.6 km h⁻¹ (1 m s⁻¹) (Silva et al. 2017, Poessel et al. 2018), we tested the analyses with both thresholds and decided to keep the speed threshold of 1 km h⁻¹ (Supporting information for a comparison of results with the two thresholds). We used the same threshold to create the proxy ‘flight time proportion’, qualifying whether the individual was in flight or perched/landed at each location of the day. The proxies of altitude, speed and ‘flight time proportion’ could not be determined for six individuals (Supporting information), as their GPS tags could not record altitude and speed. For these proxies, the dataset was therefore reduced to 78 individuals (Table 1). As the interval between two locations varied from 1 s (high-resolution with flight detection for individuals from 2019 to 2020’s cohorts) to 15 min, we resampled the whole dataset to a 15 min interval.

## Statistical analyses

All proxies (except ‘flight time proportion’) were log-transformed to limit the impact of outliers and tend toward normality. We modeled the variations in the flight skills proxies using generalized additive mixed models (GAMMs) (Zuur et al. 2009). We used a Gaussian distribution for all proxies except for ‘flight time proportion’, for which the data distribution was considered binomial. We included an individual random effect (a unique ID for each bird) in the models to take into account the non-independence of localisations from the same individual. The number of repetition of individuals on territories was too low and unbalanced to add a nested individuals/territory random effect.

For each proxy, we fitted four models to analyze the flight skills during the first 300 days since departure from the nest. The first model (‘smoothed day’ model) fit a spline smooth term for day of year using R-library mgcv (Wood 2017), with ‘individual’ as a random effect. The second model (‘sex’ model) also included a categorical predictor for ‘sex’ (male/female) and an interaction with the smooth term for day of year. The third model (‘independence’ model) also included a categorical predictor for ‘type of independence’ (early/late) and an interaction with the smooth term for day of year. The last model (‘sex_type’ model) finally included a categorical predictor for ‘sex_type’ that is a combination of both ‘sex’ and ‘type of independence’ (early males/late males/early females/late females) and an interaction with the smooth term for

day of year. Only figures of the ‘smooth day’ models and the ‘sex_type’ models are presented in the results. Figures of the ‘sex’ models and the ‘independence’ models are presented in Supporting information. We selected the best models according to the explained variance.

Note that for the ‘independence’ models, one individual (no. 180206) was removed from the datasets of all the proxies because its GPS system stopped too early to assign its independence status (‘Results’, Supporting information).

All statistical analyses were carried out using R studio software ver. 1.2.5033 (www.r-project.org), using the ‘mgcv’ library (Wood 2017). The graphics were created using the ‘ggplot2’ library (Wickham 2016).

## Results

### Duration of the PFDP

The PFDP duration lasted on average 177.9 (± 62.2) days post departure from the nest (range 44–395 days; n=84, Fig. 1). For two individuals, PFDP duration could not be calculated because their tag failed before they left their natal territory: the first one 92 days after departure from the nest (no. 180206) and the second one 211 days after the departure from the nest (no. 180214) (Supporting information). The median of the PFDP duration was 189 days: $N_{early} = 42$ and $N_{late} = 41$, and $N_{unknown} = 1$. The independence status could not be calculated for one individual (no. 180206) because his GPS system stopped too early (Supporting information). There was no significant difference in PFDP duration between females and males (Student t-test: $t=-1.70$; df=79; p=0.09). There was no significant correlation between the PFDP duration and the date of departure from the nest ($r=-0.07$, df=80, p=0.51) (Supporting information). At 300 after departure from the nest, 82 out of the 84 juveniles (about 98%) had already started the independence phase, which supporting the choice to stop the dataset at 300 days.

### Change in flight skills proxies during the first 300 days since departure from the nest

#### General pattern

The proxies ‘cumulative daily distance’ (Fig. 2B), ‘average flight height’ (Fig. 2C), ‘average speed’ (Fig. 2D) and ‘flight time proportion’ (Fig. 2E) all shared a relatively similar pattern. This pattern presented a first peak, characterized by a high and rapid increase in proxies’ values over the first 60–70 days after departure from the nest, then a rapid and relatively large decrease over the following 50–70 days followed by a plateau phase more or less pronounced depending on the proxy. Around day 180–200 after departure from the nest there was a substantial increase in the proxies’ values, which reached or exceeded the first peak and grew continuously until a new peak about 260 days after departure from the nest, after which there was a new decrease until the end of the studied period (Supporting information for ‘maximum flight height’ and ‘maximum speed’).
<figure><img src="image_7.png" /><figcaption>Figure 1. Histogram of the independence day of 84 juveniles of golden eagles determined according to the method of Weston et al. (2013). In gray, individuals with an ‘early’ independence day, or less or equal than 189 days after departure from the nest (division indicated by the black dotted line); in black, individuals with a ‘late’ independence day, or more than 189 days after departure from the nest.</figcaption></figure>

‘Maximum distance from nest’ (Fig. 2A) increased exponentially from day 180 after departure. However, compared to the other proxies, there was no marked peak around 60 days after departure from the nest (the increase in distance was very slight, followed by a plateau phase). This proxy also had a high percentage of explained variance at 49.66%, meaning with a strong effect of time since departure from the nest (Table 2).

During the first two months after departure from the nest, day after day, the juveniles flew: longer distances (‘cumulative daily distance’), farther from their nest (‘maximum distance from the nest’), more often (‘flight time proportion’), higher (height proxies) and faster (speed proxies). After 60 days, the phase in which these proxies decreased or plateau suggested that juveniles flew less and may have performed only ‘short’ flights during this period.

## Variation between sexes and type of independence

Sex had a slight effect on the temporal change in the flight skills proxies: the patterns observed for males and females were similar for six of the seven proxies (Fig. 2F–J, Supporting information). For the proxy ‘maximum distance from the nest’ (Fig. 2F), however the confidence intervals were overlapping, the patterns were very similar between both sexes up to 200 days, after which patterns diverged with higher values for females, when after the onset of dispersal females travelled considerably further than males. Nevertheless, for this proxy, the percentage of variance explained by the ‘sex’ model was relatively similar to that of the ‘smoothed day’ model (increase of < 1%), revealing that the difference were relatively small. For the six others proxies (Fig. 2G–J, Supporting information), the confidence intervals overlapped on the graphs until the first 120 days and the percentage of explained variance increased only by 1–3% compared to the ‘smoothed day’ model, indicating that the differences were not fully meaningful (Table 2). But for ‘maximum flight height’, ‘average

flight height’, ‘maximum speed’, ‘average speed’ and ‘flight time proportion’, females showed anyway lower values with no overlap with males between 120 and 220 days (and between 160 and 200 days for ‘cumulative daily distance’).

Very similar patterns for the two types of independence were found for all proxies (Fig. 2G–J, Supporting information), except for the proxy ‘maximum distance from the nest’ (Fig. 2F). In this proxy, early emigrants showed a very rapid and continuous increase in maximum distances throughout the period, but this was especially marked from day 120 after departure from the nest, while late emigrants showed a plateau or a very slight increase in maximum distances over the majority of the period, finally increasing exponentially when they left their natal territories around 180 days. This difference was expected, given that early emigrants necessarily have much higher maximum distances before 180 days, having emigrated earlier. The other differences – though significant – were small, indicating that the type of independence seems to have little or no impact on the temporal trend of the other proxies. Apart from ‘maximum distance from the nest’, in which the explained variance increased by 3.98%, the percentage of variance explained by the ‘independence’ model did not substantially increase (< 1%) compared to the ‘smoothed day’ model (Table 2).

## Discussion

We found that for 90% of the golden eagle juveniles we monitored, the PFDP lasted between 88 and 257 days, with an average duration of 177.9 days since departure from the nest. The results of our seven modeled proxies showed that flight skills seem to be acquired over the first two months after departure from the nest, a duration far shorter than total PFDP. This demonstrates that these non-migratory golden eagle juveniles remained in their natal territory long after
<figure><img src="image_8.png" /><figcaption>Figure 2. The predictions of the generalized additive mixed models (GAMMs) for five proxies analyzed according to the time since departure from the nest. Left (A) to (E): the ‘smoothed day’ model, which considered the variables individual and day effect. Right (F) to (J): the ‘sex_type’ model, which also integrated the ‘sex and type of independence’ (in interaction with the day effect). The vertical dashed line marks Day 189. The red curves represent males with late independence, the yellow curves the males with early independence, the green curves the females with late independence, and the blue curves the females with early independence. The shaded areas represent the 95% confidence intervals.</figcaption></figure>
<table><caption>Table 2. Percentage of explained variance obtained by the four generalized additive mixed models (GAMMs) according to the seven proxies used to characterize flight skills.</caption><thead><tr><th>Proxies/Models</th><th>Maximum<br>distance from<br>the nest</th><th>Cumulative daily<br>distance</th><th>Maximum<br>flight height</th><th>Average<br>flight height</th><th>Maximum<br>speed</th><th>Average<br>speed</th><th>Flight time<br>proportion</th></tr></thead><tbody><tr><td>Smoothed day</td><td>48.2%</td><td>17.3%</td><td>9.9%</td><td>8.6%</td><td>9.3%</td><td>7.8%</td><td>1.7%</td></tr><tr><td>Sex</td><td>48.9%</td><td>18.0%</td><td>11.9%</td><td>10.0%</td><td>7.8%</td><td>10.7%</td><td>2.4%</td></tr><tr><td>Independence</td><td>52.2%</td><td>17.7%</td><td>10.4%</td><td>9.0%</td><td>9.7%</td><td>8.3%</td><td>2.1%</td></tr><tr><td>Sex_Type</td><td><strong>53.3%</strong></td><td><strong>19.5%</strong></td><td><strong>12.8%</strong></td><td><strong>11.0%</strong></td><td><strong>14.0%</strong></td><td><strong>11.8%</strong></td><td><strong>2.9%</strong></td></tr></tbody></table>

The 'smoothed day' model fit a spline smooth term for day of year with 'individual' as a random effect. The 'sex' model also included a categorical predictor for 'sex' (male/female) and an interaction with the smooth term for day of year. The 'independence' model also included a categorical predictor for 'type of independence' (early/late) and an interaction with the smooth term for day of year. The 'sex_type' model finally included a categorical predictor for 'sex_type' that is a combination of both 'sex' and 'type of independence' (early males/late males/early females/late females) and an interaction with the smooth term for day of year. The explained variance with the largest percentage per variable is shown in bold.

having acquired flight skills. Our analysis also revealed that flight ability evolved independently of the dispersal strategy adopted (and only partially of sex). This reinforces the hypothesis that individuals choose to stay in their natal territory rather than being constrained to stay with their parents.

## Flight skills acquired in two months

Our results showed either a continuous or a rapid increase in the values of all the proxies describing flight skills during the first two months after departure from the nest until a peak at around 60 days, whatever the sex and the independence type. After this peak, the studied proxies were followed either by a decrease or by a plateau phase over the following 60 days. This is probably because, in the studied population when juveniles left the nest around mid-July, this period corresponds to autumn, when the weather conditions become less favourable for soaring/gliding flight due to reduced solar insolation and increased rainfall (Shamoun-Baranes et al. 2003, Shamoun-Baranes et al. 2006, Monsarrat et al. 2013, Vansteelant et al. 2015, Rousteau 2020). In our study area, thermals are weaker in autumn than in summer (Aupetit 2020), which does not allow soaring birds to fly as high. Moreover, because this period is more favourable to successes of low pressure systems, airspace conditions are more turbulent and can change more often and faster (Aupetit 2020). This requiring a better control of flight and a greater energy expenditure to do it (Shamoun-Baranes et al. 2006, Poessel et al. 2018). During this period, young golden eagles spent a lower proportion of their time in flight, and when flying, they travelled at lower height and lower groundspeed. This supports the hypothesis of a poorer flight control than experimented adults to face less favourable conditions and to move in a turbulent airspace.

These results are consistent with the literature, which shows that the flight ability of birds develops rapidly during the first weeks after departure from the nest, consequently to the acquisition of soaring flight through personal learning or public (parental) training, allowing a flight mode that is more efficient and less costly than the flapping flight practiced at departure from the nest (Bustamante and Hiraldo 1989, Duriez et al. 2014, Ruaux et al. 2020). In griffon vultures

*Gyps fulvus*, juveniles aged < 60 days are also significantly less efficient than adult birds for climbing in thermals drifted by wind (Harel et al. 2016). In the Spanish imperial eagle, soaring flight appears on average 25 days after departure from the nest and its learning is stimulated by the parents, which leave food for their offspring further and further from the nest, forcing the juveniles to fly longer distances (Ferrer 1992, Muriel et al. 2015). These results also confirm, with more numerous and detailed proxies, the results of Weston et al. (2017), which showed that golden eagles in Scotland exhibit a PFDP divided into two distinct phases: an ontogenetic phase (increased mobility), with full mobility reached on average 68 days after departure from the nest, and a relatively long maintained phase (constant mobility). That study did not find any decrease in juvenile mobility after 68 days, but only a plateau phase – possibly because the proxy they modeled was not precise enough to detect such a tiny signal. The second peak in all patterns is about 260 days after departure from the nest. At this time of the year, most of the juveniles have left their natal territory. For the studied population, this period correspond to the end of winter, when carrions are increasingly difficult to find and less nutritious due to the harshness of winter. Juveniles need to fly more and longer to forage. The last decrease in the pattern corresponds to the start of spring, with new born in prey species and foraging that become easier.

## Juveniles choose to stay in the natal territory

Our alternative hypotheses concerning the factors determining the independence of juveniles were: 1) individuals are constrained to stay in the natal territory due to limited flight skills, or 2) they choose to stay to optimize their fitness. Our results support the second hypothesis because for 90% of individuals, independence took place between 88 and 257 days after departure from the nest, well after the acquisition of flight skills within the first 60 days period after departure from the nest. This conclusion was reinforced by the fact that flight skills proxies did not differ between early and late emigrants. The juveniles appear to restrict themselves to stay in their natal territory throughout the autumn and early winter. This strategy might be the consequence of the high cost
involved in independence, especially if dispersal starts in
autumn, the season with the highest rainfall and cloud cover
in the study area (Aupetit 2020). Raptor movements are
usually highly dependent on weather conditions, as efficient
soaring flight can occur with orographic or thermal uplifts
(Duerr et al. 2015, Katzner et al. 2015, Murgatroyd et al.
2018) and precipitation greatly limit raptors' success in for-
age (Sergio 2003). Soaring flight ability as well as successful
foraging are essential for successful dispersal.

Another non-mutually exclusive postulate could be that juveniles continue to be fed by the parents, as they have not yet acquired the hunting skills and/or sufficient body condition to support the costs of dispersal. Yet in golden eagles, depending on food availability in the natal territory, parents stop feeding their young about six weeks after fledging (O'Toole et al. 1999 for migratory eagles in Missouri) to eleven weeks after fledging (Watson 2010 for resident eagles in Scotland). It may be possible that even if the adults no longer directly feed the juveniles, they may guide them (directly or indirectly) to carrion during winter. Photos from camera-traps close to carrion in winter commonly show juveniles and adults together, hence strongly suggesting support for this hypothesis (Supporting information). Juveniles may also use this period to continue to learn how to hunt with their parents. In future studies, it would be relevant to simultaneously track juveniles and parents to get more insight into adult/ juvenile interactions during this phase. It is also possible that the knowledge of the natal territory acquired by juveniles during the first weeks of exploration guarantees easy access to food resources in contrast to surrounding unknown territories, therefore encouraging them to stay in natal territory. In surrounding territories, juveniles would have to compete with other territorial adults, which would represent an additional cost (or threat).

Thus, the costs of leaving the natal territory early seem potentially high, and the benefits of dispersal minimal. Golden eagles generally acquire their breeding territory between 4 and 6 years old (Watson 2010), so there seems little benefit to gain by dispersing a few days or weeks earlier. This hypothesis is reinforced by the fact that although females disperse farther than males during the dispersal phase, they do not emigrate earlier. The farther dispersal of females has been noted in other golden eagle studies (Soutullo et al. 2006) and is consistent with the general pattern that female birds tend to disperse farther than males (Greenwood 1980, Clarke et al. 1997, but see in Mabry et al. (2013) for rare references on male-biased dispersal in bird species). Despite the likely benefit of farther dispersal, females stay just as long in the natal territory as males. Nevertheless, the period between 120 days and 220 days after departure from the nest, period corresponding to winter with weaker thermals, females had lower flight skills than males. Since females are heavier, they have a less good buoyancy than males under the same aerological conditions and therefore spend less time in flight (Alarcon et al. 2017, Gonzalez-Solis et al. 2000). It would be interesting in future studies to investigate the post-dispersal period in more details.

Finally, we cannot exclude the hypothesis that the proxies we studied may only partially reflect the acquisition of flight abilities, and that independence is constrained by other unstudied flight skills. It would be interesting in future studies to investigate in more detail the acquisition of different types of flight, such as behavior in thermals studied in grifon vultures (Harel et al 2016), or the interactions between the acquisition of soaring flight and hunting skills in great frigate birds (Corbeau et al. 2020), the latter probably taking more time to acquire and yet crucial for juvenile survival. It would also be useful to examine new proxies, for example, based on accelerometry and magnetometry with high-resolution data, combined with speed and altitude data (Williams et al. 2015), as well as to take advantage of movements recorded in 1 s interval of individuals in 2019 and 2020 in our study.

Departure from natal territory depends on other factors that may influence parental decisions or juvenile personality

Our results suggest that it is advantageous for juveniles to stay in their natal territory for longer than strictly necessary in terms of flight constraints in order to maximize their fitness. This behavior implies that adults tolerate juveniles in their territory and therefore accept the cost of sharing food resources. That is probably what happen for raptors species with a long PFDP duration like harpy (Muñiz-Lopez et al. 2012, Urios et al. 2017) or Philippine eagles (Clark et al. 2020), which then breed only one year out of two. This in turn implies that adults' tolerance of juveniles in the territory increases their own fitness, by increasing the survival of their offspring without reducing their own survival. The juvenile golden eagles studied had a PFDP lasting between 44 and 395 days, showing high inter-individual variability. These results are similar to those recorded in golden eagles in Scotland (Weston et al. 2013), however, they contrast with previous studies in golden eagles in Spain (PFDP between 60 and 120 days; Soutullo et al. 2006) and in North America for a migrant population (between 39 and 63 days; McIntyre and Collopy 2006). In two related species, the Spanish imperial eagle (Muriel et al. 2015, Ramos et al. 2019) and Bonelli's eagle *Hieraaetus fasciatus* (Real et al. 1998, Balbontín and Ferrer 2005, Cadahía et al. 2008), the PFDP has also been found to be shorter, ranging from 25 to 115 days after departure from the nest. The short duration of this period seems logical for the population of golden eagles studied by McIntyre and Collopy (2006), which migrate and are thus constrained by the migration departure, but less so for the sedentary Spanish golden eagle population as well as the other eagle species mentioned above. This difference could be explained by the fact that the population in Spain is not subject to the same weather and food constraints as the population in France, and thus developed a different dispersal strategy. It seems plausible that in regions with significant food resources and mild climatic conditions in autumn/winter, parents may tend to expel juveniles more quickly outside
of their territory as their chances of survival may be relatively high (Trivers 1974, Alonso et al. 1987).

However, this would not explain the variability in this period between individuals of the same population, as found in our study. Individual decisions and behaviours impact movements within a studied population (Dingemanse et al. 2010, Sih et al. 2012, Spiegel et al. 2017, Hertel et al. 2020). Personality differences such as boldness or shyness are known to contribute to variations in many behaviour such as foraging or decision-making (Réale et al. 2007, Patrick et al. 2017). We posit that the decision to emigrate may be initiated independently of the parents by juveniles that are ‘bolder’ or in good body condition, which would more quickly leave the natal territory, as opposed to juveniles that are ‘shyer’ or in poor body condition, which would tend to remain as long as possible in the territory until exclusion by the parents.

An alternative hypothesis may be that adults exclude juveniles more or less quickly depending on the quantity of food resources available in their own territory and depending on their investment in a new breeding attempt. The long-term monitoring program of golden eagles in France would make it possible to test these hypotheses in the future by studying inter-annual variability in PFDP duration in the same territory (i.e. between juveniles of the same parents over the years).

## Conclusion

Despite the fact that the PFDP is considered as one of the most critical stages in the lifecycle of raptors – and birds in general – our knowledge of this period and the factors shaping it are still very patchy (Delgado et al. 2009, López-López et al. 2014, Weston et al. 2017). The few studies dealing with this, including our study, show high heterogeneity in independence timing between juveniles within the same population, and between populations of conspecifics or related species. In the future, it would be valuable to study the ultimate and proximate factors that lead to such variability in PFDP duration. As flight and foraging skills are intrinsically linked, it seems important to study in detail the acquisition of different flight modes and foraging performances, which is now possible thanks to the availability of high-resolution tracking technology such as accelerometry and magnetometry (Harel et al. 2016, Corbeau et al. 2020). It also seems worthwhile to study the length of this period in relation not only to juveniles but also parents, which could also be done with high-resolution tracking, but would involve the challenging deployment of GPS tracking devices on both parents and juveniles in the same territory.

*Acknowledgements* – This project was partially funded (for the Hautes-Alpes dataset) by “Réseau Transport d’Electricité” that we would like to thank. We also would like to thank the numerous people involved in this program that helped us in the field to monitor the populations and capture the individuals: officers of

Cévennes' and Ecrins' Natl Parcs, of French Office of Biodiversity, volunteers of many naturalists NGO, but also unaffiliated volunteers, climbers and mountain guides, etc. We also would like to thank J. Shamoun-Baranes, Todd Katzner and an anonymous reviewer for their valuable advices and comments.

*Funding* – This work was in part (study in the Alps) funded by “Réseau Transport d’Electricité”.

*Permits* – Eagles were manipulated and equipped following national regulation scheme. In this context the program was authorized by the national authorities (Musuem Natl d’Histoire Naturel) through the “Personal Banding Program” no. 579.

## Author contributions

**Arzhela Hemery:** Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – review and editing (equal). **Lucas Mugnier-Lavorel:** Data curation (equal); Formal analysis (equal); Methodology (equal); Writing – original draft (equal). **Christian Itty:** Conceptualization (equal); Data curation (equal); Investigation (equal); Resources (equal); Writing – review and editing (equal).

**Olivier Duriez:** Methodology (equal); Supervision (equal); Writing – review and editing (equal). **Aurelien Besnard:** Conceptualization (equal); Formal analysis (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review and editing (equal).

## Transparent peer review

The peer review history for this article is available at https://publons.com/publon/10.1111/jav.03111.

## Data availability statement

Data are available from the Zenodo Digital Repository: https://doi.org/10.5281/zenodo.7802538 (Hemery et al. 2023).

All data used in this study are available upon detailed request to data manager Christian Itty in the online data repository Movebank (www.movebank.org). The projects are: “Aquila Chrysaetos Golden Eagle France/Central massif [ID_PROG 579” (project ID=137262099), “Aquila chrysaetos Golden Eagle France/Drome [ID_PROG 579]” (project ID=509061405), and “Aquila Chrysaetos Golden Eagle France/Hautes Alpes [ID_PROG 579]” (project ID=431257563).

## Supporting information

The Supporting information associated with this article is available with the online version.

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