that this bearded vulture population may have suffered a process of territorial
compression associated with an increase in the number of breeding pairs (nearly
25% reduction in the mean nearest neighbour distance between 1987 and 2002,
Margalida et al. 2008a). This fact may be affecting the productivity of the population (see Carrete et al. 2006a). Moreover, Donázar et al. (1993) did not find any
relationship between breeding success and distance to conspecific breeding pairs,
suggesting that productivity was not limited by any density-dependent mechanism
before 1991. Consequences of territory compression have been explored in other
species, where increases in density are accompanied by increases in aggressive
behaviour among territorial animals and costs associated with territory defence (e.g.
Calsbeek and Sinervo 2002; Mougeot et al. 2003; Sillett et al. 2004). In these cases,
territory shrinkage and territorial disputes associated with high-density situations
affected reproduction and had fitness costs for territorial animals (Gordon 1997;
Calsbeek and Sinervo 2002; Ridley et al. 2004), as in our bearded vulture population. However, we found that not all territories were equally affected by increases
in the number of breeding pairs. Territories located in high-density situations
became less productive and more unpredictable than territories located far away
from conspecific pairs, indicating that in the present situation proximity to other
breeding pairs could be the main factor promoting territory quality and also, to
some extent, habitat heterogeneity in this closed population.
15.5.3 Effects of Supplementary Feeding Sites
on Breeding Output
Proximity to supplementary feeding points where non-breeding birds congregate
was also detrimental for reproduction (Fig. 15.3). For species with delayed maturity
such as many long-lived raptors, spatial segregation between dispersing and
breeding birds is a common feature (Newton 1979). This is because preparation for
reproduction governs preferences among breeders, while food is the main driving
force underlying habitat selection patterns in dispersing birds (e.g. Bustamante et al.
1997; Brown 1997; Mañosa et al. 1998; Hirzel et al. 2004). In our study area,
however, the high availability of food resources associated with supplementary
feeding points allows a great number of non-breeding bearded vultures—which
otherwise would be occupying different areas (Brown 1997; Xirouchakis and
Nikolakalis 2002; Hirzel et al. 2004; Margalida et al. 2011a)—to coexist within the
spatial distribution of the breeding population. Contrary to the social behaviour
observed in other species in which floaters and territorial birds may coexist in areas
of high food supply (e.g. Blanco and Tella 1999), bearded vultures are territorial
birds that defend exclusive breeding areas against both conspecifics and
heterospecific birds (e.g. Bertran and Margalida 2002b; Margalida and Bertran
2000, 2005). Thus, high concentrations of floaters around breeding territories—as
356
A. Margalida
compression associated with an increase in the number of breeding pairs (nearly
25% reduction in the mean nearest neighbour distance between 1987 and 2002,
Margalida et al. 2008a). This fact may be affecting the productivity of the population (see Carrete et al. 2006a). Moreover, Donázar et al. (1993) did not find any
relationship between breeding success and distance to conspecific breeding pairs,
suggesting that productivity was not limited by any density-dependent mechanism
before 1991. Consequences of territory compression have been explored in other
species, where increases in density are accompanied by increases in aggressive
behaviour among territorial animals and costs associated with territory defence (e.g.
Calsbeek and Sinervo 2002; Mougeot et al. 2003; Sillett et al. 2004). In these cases,
territory shrinkage and territorial disputes associated with high-density situations
affected reproduction and had fitness costs for territorial animals (Gordon 1997;
Calsbeek and Sinervo 2002; Ridley et al. 2004), as in our bearded vulture population. However, we found that not all territories were equally affected by increases
in the number of breeding pairs. Territories located in high-density situations
became less productive and more unpredictable than territories located far away
from conspecific pairs, indicating that in the present situation proximity to other
breeding pairs could be the main factor promoting territory quality and also, to
some extent, habitat heterogeneity in this closed population.
15.5.3 Effects of Supplementary Feeding Sites
on Breeding Output
Proximity to supplementary feeding points where non-breeding birds congregate
was also detrimental for reproduction (Fig. 15.3). For species with delayed maturity
such as many long-lived raptors, spatial segregation between dispersing and
breeding birds is a common feature (Newton 1979). This is because preparation for
reproduction governs preferences among breeders, while food is the main driving
force underlying habitat selection patterns in dispersing birds (e.g. Bustamante et al.
1997; Brown 1997; Mañosa et al. 1998; Hirzel et al. 2004). In our study area,
however, the high availability of food resources associated with supplementary
feeding points allows a great number of non-breeding bearded vultures—which
otherwise would be occupying different areas (Brown 1997; Xirouchakis and
Nikolakalis 2002; Hirzel et al. 2004; Margalida et al. 2011a)—to coexist within the
spatial distribution of the breeding population. Contrary to the social behaviour
observed in other species in which floaters and territorial birds may coexist in areas
of high food supply (e.g. Blanco and Tella 1999), bearded vultures are territorial
birds that defend exclusive breeding areas against both conspecifics and
heterospecific birds (e.g. Bertran and Margalida 2002b; Margalida and Bertran
2000, 2005). Thus, high concentrations of floaters around breeding territories—as
356
A. Margalida