the Pyrenees (Margalida et al. 2011a). With respect to non-natural mortality,
although mortality factors affect mostly adult age classes (Margalida et al. 2007b;
Oro et al. 2008), this seems to play no role in the start of breeding. Thus, buffering
capacity through ecological and behavioural processes such as positive feedback
from conspecific attraction could explain this regular pattern (Margalida et al.
2011d; Oro et al. 2012).
During the last six years, the proportion of double clutches seemed to decrease in
parallel with laying dates. Large avian species often show obligate brood reduction
(two eggs laid but only one chick survives) in which siblicide (when more that 90%
of last-hatched chicks die, Simmons 1988) may occur, even when food supplies are
abundant. The second egg thus serves as insurance against loss of the first egg from
infertility, predation, or damage, rather than as a means of rearing two chicks
(Stinson 1979; Anderson 1990; Mock et al. 1990). According to Winkler et al.
(2002), there is a strong effect of laying date on clutch size, and earlier breeding
may lead to larger clutch sizes. An increase of food resources could increase clutch
size in fratricide facultative species (Korpimaki and Wiehn 1998), but in obligate
fratricide species such as bearded vultures, the second egg seems to be an adaptive
mechanism serving as an insurance egg to avoid breeding failure if the first egg
does not hatch (Stinson 1979; Margalida et al. 2004). In this respect, in several
species, including the bearded vulture, it seems that single-egg clutches are related
to old or young females (Frey et al. 1995). According to our results, the increase of
single-egg clutches could be related to a reduction in food supply or the stress that
changes in food availability could provoke in individuals (i.e. increasing their
foraging effort or the agonistic interactions as a consequence of a greater competence for the resources) and the effects of mate loss with the incorporation of less
experienced individuals.
15.7.4 The Impact of Policies Decisions on Breeding
Ecology and Conservation
The conservation implications of these results suggest that anthropogenic activities
modifying habitat quality through human health regulations (i.e. a reduction in food
availability) can alter demographic parameters and increase the probability of illegal
poison bait consumption thus increasing non-natural mortality. The shifts in
demographic parameters can have important conservation consequences, being
necessary to adopt preventive mitigation measures on targeted species. As a result,
preventive management measures such as supplementary feeding can be effective
until more flexible sanitary legislation allows increased trophic availability, minimising the detrimental effects that food shortages and illegal poisoning can cause.
374
A. Margalida
although mortality factors affect mostly adult age classes (Margalida et al. 2007b;
Oro et al. 2008), this seems to play no role in the start of breeding. Thus, buffering
capacity through ecological and behavioural processes such as positive feedback
from conspecific attraction could explain this regular pattern (Margalida et al.
2011d; Oro et al. 2012).
During the last six years, the proportion of double clutches seemed to decrease in
parallel with laying dates. Large avian species often show obligate brood reduction
(two eggs laid but only one chick survives) in which siblicide (when more that 90%
of last-hatched chicks die, Simmons 1988) may occur, even when food supplies are
abundant. The second egg thus serves as insurance against loss of the first egg from
infertility, predation, or damage, rather than as a means of rearing two chicks
(Stinson 1979; Anderson 1990; Mock et al. 1990). According to Winkler et al.
(2002), there is a strong effect of laying date on clutch size, and earlier breeding
may lead to larger clutch sizes. An increase of food resources could increase clutch
size in fratricide facultative species (Korpimaki and Wiehn 1998), but in obligate
fratricide species such as bearded vultures, the second egg seems to be an adaptive
mechanism serving as an insurance egg to avoid breeding failure if the first egg
does not hatch (Stinson 1979; Margalida et al. 2004). In this respect, in several
species, including the bearded vulture, it seems that single-egg clutches are related
to old or young females (Frey et al. 1995). According to our results, the increase of
single-egg clutches could be related to a reduction in food supply or the stress that
changes in food availability could provoke in individuals (i.e. increasing their
foraging effort or the agonistic interactions as a consequence of a greater competence for the resources) and the effects of mate loss with the incorporation of less
experienced individuals.
15.7.4 The Impact of Policies Decisions on Breeding
Ecology and Conservation
The conservation implications of these results suggest that anthropogenic activities
modifying habitat quality through human health regulations (i.e. a reduction in food
availability) can alter demographic parameters and increase the probability of illegal
poison bait consumption thus increasing non-natural mortality. The shifts in
demographic parameters can have important conservation consequences, being
necessary to adopt preventive mitigation measures on targeted species. As a result,
preventive management measures such as supplementary feeding can be effective
until more flexible sanitary legislation allows increased trophic availability, minimising the detrimental effects that food shortages and illegal poisoning can cause.
374
A. Margalida