The relevance of SFS for population viability through increments in the survival
rates of non-territorial birds challenges the theoretical low sensitivity of population
growth rates to this parameter expected among long-lived species (Saether and
Bakke 2000). Conversely, SFS appear to be discouraging population expansion
outside the Pyrenees and have been related to habitat saturation processes
(Margalida et al. 2008a), which are triggering negative population effects such as
reductions in productivity and changes in mating behaviour (Carrete et al. 2006a).
Taking into account the life-history characteristics of the species and based on the
general hypothesis of positive effects of predictable food supply on survival, we
predict that increments in the illegal use of poison during recent decades
(Hernández and Margalida 2008, 2009; Margalida 2012; Mateo-Tomás et al. 2012)
have decreased survival rates of bearded vultures. However, this effect would have
been buffered by the use of SFS, the use of which varies considerably among
individuals although a general pattern is associated with age (young birds are more
frequently seen at SFS than older ones, Margalida et al. 2011c). Population projections under different management scenarios in which poisoning have been
combated with a variable degree of effectiveness are also presented.
15.6.1 Relationship Between Supplementary Feeding
and Survival
We assessed the role of supplementary feeding on bearded vulture’s survival. The
best survival model obtained using the MARK program accounts for 85% of the
weight of all models and included the effects of age (young or adults), time and
the intensity of use of SFS on survival, and of age on recapture rates. Interestingly, the
addition of the individual covariate describing the intensity of use of SFS significantly
improved the models. However, the use of SFS decreased with bird age, even after
controlling for individual variability (see Oro et al. 2008). Thus, survival was equal
for birds of ages up to 4 years old (the age classes more frequently seen at SFS) and
remained constant over time (0.944) even after the recent increment in the illegal use
of poison. Conversely, survival of older birds ( ! 5 years old) decreased linearly with
time (Fig. 15.4), with an average value of 0.878. Other models, including a higher
number of age classes (results not shown) or different combinations of age and time,
behaved worse. Thus, the results support the hypothesis of improvement of survival
rates associated with the use of SFS. Indeed, survival estimates for the two most
important age classes at the beginning of the study period (1987) when the use of
illegal poison was less marked, but SFS were not available, were 0.787 and 0.961 for
young (the age class using SFS) and adults, respectively.
Ecological theory predicts that survival of long-lived species should increase
gradually with age because (1) individuals improve their competitive skills or
change from vagrant lifestyles to more sedentary ones after territory acquisition
15 Importance of Long-Term Studies to Conservation …
359
rates of non-territorial birds challenges the theoretical low sensitivity of population
growth rates to this parameter expected among long-lived species (Saether and
Bakke 2000). Conversely, SFS appear to be discouraging population expansion
outside the Pyrenees and have been related to habitat saturation processes
(Margalida et al. 2008a), which are triggering negative population effects such as
reductions in productivity and changes in mating behaviour (Carrete et al. 2006a).
Taking into account the life-history characteristics of the species and based on the
general hypothesis of positive effects of predictable food supply on survival, we
predict that increments in the illegal use of poison during recent decades
(Hernández and Margalida 2008, 2009; Margalida 2012; Mateo-Tomás et al. 2012)
have decreased survival rates of bearded vultures. However, this effect would have
been buffered by the use of SFS, the use of which varies considerably among
individuals although a general pattern is associated with age (young birds are more
frequently seen at SFS than older ones, Margalida et al. 2011c). Population projections under different management scenarios in which poisoning have been
combated with a variable degree of effectiveness are also presented.
15.6.1 Relationship Between Supplementary Feeding
and Survival
We assessed the role of supplementary feeding on bearded vulture’s survival. The
best survival model obtained using the MARK program accounts for 85% of the
weight of all models and included the effects of age (young or adults), time and
the intensity of use of SFS on survival, and of age on recapture rates. Interestingly, the
addition of the individual covariate describing the intensity of use of SFS significantly
improved the models. However, the use of SFS decreased with bird age, even after
controlling for individual variability (see Oro et al. 2008). Thus, survival was equal
for birds of ages up to 4 years old (the age classes more frequently seen at SFS) and
remained constant over time (0.944) even after the recent increment in the illegal use
of poison. Conversely, survival of older birds ( ! 5 years old) decreased linearly with
time (Fig. 15.4), with an average value of 0.878. Other models, including a higher
number of age classes (results not shown) or different combinations of age and time,
behaved worse. Thus, the results support the hypothesis of improvement of survival
rates associated with the use of SFS. Indeed, survival estimates for the two most
important age classes at the beginning of the study period (1987) when the use of
illegal poison was less marked, but SFS were not available, were 0.787 and 0.961 for
young (the age class using SFS) and adults, respectively.
Ecological theory predicts that survival of long-lived species should increase
gradually with age because (1) individuals improve their competitive skills or
change from vagrant lifestyles to more sedentary ones after territory acquisition
15 Importance of Long-Term Studies to Conservation …
359