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.