cliffs and randomly selected cliffs. Therefore, during the last years, bearded vultures
have been selecting a similar kind of cliff as previously chosen in relation to
topography and distance from human habitation, the main factors affecting cliff
selection. The distance between neighbouring territories decreased in parallel with
the increase in population density (from 4.4 to 2.5 territories/1000 km
2 during
1991–2002).
15.3.3 Why Bearded Vultures Do not Extend Their
Distribution Range?
Our study suggests that territory compression may occur without serious modification of nesting habitat quality. Our findings show that the selection of nest sites
has not changed with the population increase of the Pyrenean bearded vulture. For
those variables determining the probability of cliff occupancy (ruggedness, altitude
and distance to the nearest human habitation), newly established territories between
1991 and 2002 had similar values to those of traditional territories occupied previously. These new pairs, however, occupied cliffs in between the existing old
territories so that the distance between neighbours has reduced by around 20%,
which proves territory shrinkage (Both and Visser 2000; Ridley et al. 2004). Cliffs
remaining unoccupied by bearded vultures after 1991 in the periphery of the
Pyrenees had both lower values in key variables (e.g. relief, altitude, distance to a
village) and lower probability of occupancy than those occupied in the Pyrenean
range during the same period. These findings seem to contradict what would be
expected from a distribution by despotic competition, classically described for birds
of prey and other territorial birds (Newton 1998). It could be argued that there is
still availability of good-quality breeding sites in the Pyrenees and that, consequently, the population has not reached the saturation threshold necessary to
observe the occupation of marginal territories by newly established birds. This
argument holds true if the comparison is limited to the examination of the quality of
nesting cliffs where no long-term changes were appreciable. A different picture,
however, arises when the distance between neighbouring territories is considered.
Since this variable was significant in the model fitted in 1991, we can deduce that
saturation is taking place and that there is a decrease in territorial quality via a
reduction in the distance between breeding pairs. In parallel, it has been demonstrated that the productivity of the Pyrenean population has sharply declined during
the last decade as a consequence of the increasing breeding density (Carrete et al.
2006a; Margalida et al. 2014b), a clear symptom of population crowding, as is the
appearance of unusual mating systems (Bertran and Margalida 2003; Carrete et al.
2006b; Bertran et al. 2009).
The packing process could also be motivated by the scarcity of suitable breeding
places in mountain areas surrounding the Pyrenees. As our analyses demonstrate,
potential nesting cliffs in those regions are of lower quality (regarding altitude,
348
A. Margalida
have been selecting a similar kind of cliff as previously chosen in relation to
topography and distance from human habitation, the main factors affecting cliff
selection. The distance between neighbouring territories decreased in parallel with
the increase in population density (from 4.4 to 2.5 territories/1000 km
2 during
1991–2002).
15.3.3 Why Bearded Vultures Do not Extend Their
Distribution Range?
Our study suggests that territory compression may occur without serious modification of nesting habitat quality. Our findings show that the selection of nest sites
has not changed with the population increase of the Pyrenean bearded vulture. For
those variables determining the probability of cliff occupancy (ruggedness, altitude
and distance to the nearest human habitation), newly established territories between
1991 and 2002 had similar values to those of traditional territories occupied previously. These new pairs, however, occupied cliffs in between the existing old
territories so that the distance between neighbours has reduced by around 20%,
which proves territory shrinkage (Both and Visser 2000; Ridley et al. 2004). Cliffs
remaining unoccupied by bearded vultures after 1991 in the periphery of the
Pyrenees had both lower values in key variables (e.g. relief, altitude, distance to a
village) and lower probability of occupancy than those occupied in the Pyrenean
range during the same period. These findings seem to contradict what would be
expected from a distribution by despotic competition, classically described for birds
of prey and other territorial birds (Newton 1998). It could be argued that there is
still availability of good-quality breeding sites in the Pyrenees and that, consequently, the population has not reached the saturation threshold necessary to
observe the occupation of marginal territories by newly established birds. This
argument holds true if the comparison is limited to the examination of the quality of
nesting cliffs where no long-term changes were appreciable. A different picture,
however, arises when the distance between neighbouring territories is considered.
Since this variable was significant in the model fitted in 1991, we can deduce that
saturation is taking place and that there is a decrease in territorial quality via a
reduction in the distance between breeding pairs. In parallel, it has been demonstrated that the productivity of the Pyrenean population has sharply declined during
the last decade as a consequence of the increasing breeding density (Carrete et al.
2006a; Margalida et al. 2014b), a clear symptom of population crowding, as is the
appearance of unusual mating systems (Bertran and Margalida 2003; Carrete et al.
2006b; Bertran et al. 2009).
The packing process could also be motivated by the scarcity of suitable breeding
places in mountain areas surrounding the Pyrenees. As our analyses demonstrate,
potential nesting cliffs in those regions are of lower quality (regarding altitude,
348
A. Margalida