We report on a 32—yr study of a population of Spanish Imperial Eagles, Aquila adalberti, which increased during the first 16 yr of study but remained stable during the last 16 yr. We analyzed changes in the mean and variance of fecundity in relation to population density to test predictions of two hypotheses of density—dependent fecundity. According to the interference" hypothesis, as density increases, frequency of agonistic encounters increases, resulting in a relatively uniform decrease in habitat quality. Consequently, mean fecundity decreases, and no relationship is expected between density and variance in fecundity. For the habitat heterogeneity hypothesis, however, as density increases, a greater proportion of individuals are forced to occupy lower quality habitats. Thus, mean fecundity decreases and fecundity variance must increase. Additionally, for this hypothesis, fecundities in good sites are expected to be equal in both low— and high—density situations. An inverse relationship between fecundity and population size was found in this eagle population. Annual variance in productivity showed significant increases over the study period. This trend was inversely related to mean productivity. Variance in productivity was related to the year of pair establishment, being higher in recently occupied territories. Mean and variance of the longer term territories remained constant during the study period. These results are in accordance with the habitat heterogeneity hypothesis.
the species in the study area, did not differ from other fish species (hake) in its δ 15 N value, we used it to represent a fish dietary alternative. Using a 2-source (anchovy and squid) isotopic mixing model, we determined that the mean proportion of anchovy in the diet was 49% for yearlings, 76% for chicks, and 69% and 67% for adult males and females, respectively. Sex and age differences in diet, as revealed by stable isotopes, may be the consequence of individual morphology (sexual size dimorphism) and reproductive constraints imposed by chick development since growing young require more nutritive prey than adults and yearlings. This reasoning would also explain the significant and positive correlation found between proportion of anchovy in the diet and body condition of chicks.
Despite its key role in population dynamics and evolutionary ecology, little is known about factors shaping survival in long-lived territorial species. Here, we assessed several hypotheses that might explain variability in survival in a migratory Spanish population of a long-lived territorial species, the Egyptian vulture Neophron percnopterus, using a 16-year monitoring period and live-encounter histories of 835 individually marked birds. Cormack-Jolly-Seber captureÁrecapture models showed no evidence for effects of sex or nestling body condition on survival. However, the normalized difference vegetation index (NDVI; an indicator of primary productivity) of natal territories had positive effects on juvenile survival, indicating that environmental conditions experienced early in life can determine survival prospects. Survival increased with age (0.7390.02 in the first 2 years to 0.7890.03 in years 3 and 4) to later decrease when birds were five years old (0.6090.05), the age at which they acquire the adult plumage, abandon the communal lifestyle of juveniles, and may look for a breeding territory. At older ages, survival was higher for non-breeding (0.7590.02) and breeding adults (0.8390.02). Among the latter, birds that recruited into better territories had higher survival prospects. Age-specific variation in survival in this species may be related to behavioural changes linked to dispersal and recruitment into the breeding population, while survival prospects of adult birds strongly depend on breeding territory selection. These results suggest a tradeoff between recruiting soon, and thus reducing mortality costs of a long and extensive dispersal period, and trying to recruit into a good quality territory. Finally, annual survival rates for birds of all age classes were positively related with the NDVI in their African wintering grounds. Although this relationship was probably mediated by food availability, further research is needed to properly identify the limiting factors that are affecting trans-Saharan migrants, especially in light of global climate change.
Causes of natal dispersal were studied in an isolated population of the migratory, facultatively colonial Lesser Kestrel (Falco naumanni) in northeastern Spain. During a seven‐year study, we gathered information on natal dispersal of 751 individuals and on 24 explanatory variables that evaluated individual traits, conspecific cues in terms of colony size, and different ecological and populational features. We examined separately whether or not individuals changed colonies and how far they moved. Dispersal from the natal colony was high (83%), and dispersers covered a median distance of 7225 m (range 112–136 500 m). The probability of natal dispersal decreased with the size of the natal colony and with the distance to the nearest colony, but not with the distance to unoccupied buildings, in the year of recruitment. Moreover, the shorter the distance to the nearest colony, the shorter the distances that individuals dispersed, particularly to large colonies (>10 pairs). These results support the conspecific attraction hypothesis. Accordingly, the probability of recruiting in the natal colony increased with the proportion of philopatric adults, although beyond a threshold individuals were prevented from recruiting by the more dominant adults in a despotic way. The timing of arrival from the wintering grounds was positively related to the probability of colony change, especially in males. To the extent that this variable is related to the phenotypic quality of individuals, it reinforces the idea that natal dispersal is shaped by intraspecific competition in crowded colonies. Males showed lower frequencies of colony change and dispersed shorter distances than did females, as explained by the different role of the sexes in nest acquisition and defense. Our results indicate that natal dispersal is an evolutionarily labile trait simultaneously influenced by ecological, social, and individual features both within and between populations.
Managing forests to meet both production and conservation goals is a growing challenge around the world, particularly as forest areas devoid of human intervention continue to shrink in area. There is a general lack of information about the response of long‐lived vertebrates to silvicultural treatments due to the difficulty of carrying out long‐term studies that can disentangle the effects of forestry from other factors influencing the distribution and numbers of large organisms. We examined the responses of the Cinereous Vulture, Aegypius monachus, to two divergent forestry practices in two colonies where ∼10% of the European population breeds: (1) long‐term exploitation of mature forest in high mountains (highland colony), and (2) clear‐cutting and regeneration in lower altitude mountain areas (lowland colony). Distribution patterns of Cinereous Vultures were determined mainly by forestry and human‐related activities, as well as by orographic variables, whereas forest structure had a lesser influence. Ecological factors such as orography and forest structure had a greater influence in the distribution and abundance of vultures in the highland area, whereas the influence of variables associated with human disturbance was stronger in the lowland area. Vultures in the lowland area consistently selected areas with a low incidence of forestry activity to breed. A major fraction of the variability of breeding success was not explained by the factors considered, although human‐caused disturbance and rainfall during the nestling period had negative effects on breeding success in the lowland colony. The negative effects of forestry activities may persist for a long time, even after the direct impacts of these activities have diminished. In order to effectively conserve breeding areas, it may be necessary to protect suitable habitat from human activity near nests. In addition, halting illegal poisoning is a strict necessity to guarantee the conservation of colonies. Social conflict between conservationists and forest managers who employ local people in nonproductive forestry tasks could easily be resolved by conducting silviculture‐related practices in areas that are unimportant for conservation and directing employment toward activities related to nature conservation.
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