4. DISCUSSION
Griffon Vulture movements varied between seasons, breeding regions and sexes. We found that movements were larger during spring and summer, which is similar to other soaring raptors such as Bearded Vulture or Bonelli’s Eagle (see Margalida et al., 2016; Pérez-García et al., 2013). This could be related to the food requirements associated with reproduction, which may force vultures to prospect larger areas, seeking for dispersed and unpredictable carcasses to satisfy the food requirements of the offspring and themselves (Carrete & Donázar, 2005). However, seasonal differences could be also explained by longer days (and therefore more time to forage) and better atmospheric conditions particularly during summer, minimising energy expenditure during long-range movements (see Martín Díaz et al., 2020). Similarly, differences in monthly home-range size and cumulative distance travelled between regions could be explained by differences in both, the importance of predictable and non-predictable food availability and the ability to move due to better flight conditions due to the presence of thermal uplifts (Scacco et al., 2021). For instance, the southern populations may experience higher thermal uplift availability (mainly due to warmer climatic conditions in summer), thus minimizing the energy expenditure while increasing the movement capacity of birds (e.g., see Scacco et al., 2019).
Differences in foraging performance between sexes are common in mammals and birds, and are often due to differences in body size and parental duties (Lewis et al., 2002), In monomorphic species such as the Griffon Vulture, these differences might be associated with energetic and nutritional requirements for reproduction (Pinet et al., 2012; Bennison et al., 2022). Our results showed that females have larger home-ranges and travel farther than males. These findings align with other studies in which the same dataset was analysed and where females exhibited larger travelled distances than males during the reproduction (see Delgado-González et al.,2022; Gangoso et al., 2021), including other vulture species (see Bamford et al., 2007; Kruger et al., 2014; Kane et al., 2015, Margalida et al.,2016; García-Jiménez et al., 2018).
Contrary to our expectations, we also found sex differences in the monthly home-ranges fidelity. Males showed greater fidelity than females, indicating that the latter use different areas throughout the year. This gender variation in seasonal fidelity may respond to differences in foraging efficiency or, rather, to resource selectivity (Hertel et al., 2020; Delgado-González et al., 2022). In fact, according to Fernández-Gómez et al. (2022) males may be more prone to feed on predictable resources such as supplementary feeding stations or vulture restaurants, while females may rely on more ephemeral and less clumped food resources. Thus, there may be parallel strategies in the large-scale exploitation of space and, therefore, sexual spatial segregation (Perrig et al., 2021). Alternatively, the fact that males show a greater fidelity of their monthly home-ranges throughout the year and that it is higher during the spring-summer period could be related to their greater territoriality. Males may be more involved in nest guarding (Kokko & Morrell, 2005) in response to competence for nesting sites (see Taborsky, 2021). Interestingly, females not only showed higher extension of home-ranges than males, but also exhibited lower site fidelity. All this reinforces the argument that females might forage more in different locations throughout the year, therefore showing lesser home-range fidelity over time than males. These sex and seasonal differences in individual home range fidelity were similar between breeding regions except in the case of Cadiz and Segovia, where females exhibited lowers fidelity during spring/summer.
Differences (or lack of them) between breeding regions in individual movement patterns indicate that local effects not explored in this work may be affecting them. This is evidenced by the low variance explained by the fixed factors of the best models and the high variance explained by random factors (e.g., home range size fidelity models). The latter indicate that high inter-individual differences in home range area and fidelity exists. Moreover, there are variables such as distance to trophic resources, colony size or habitat type that perhaps could improve the results from our models and better explain breeding region level differences in the evaluated parameters (see Harel et al., 2017; Cecere et al., 2018; Delgado-González et al., 2022). Finally, it should be taken into account that the effect of differences in sampling duration of individuals of some breeding regions also affects the observed home range sizes which may have led to an underestimation of this and other parameters such as cumulative distance or home range fidelity.