A long-term large-scale study of the breeding biology of the Spanish imperial eagle (Aquila adalberti)

Abstract

We present data from a 17-year study of the population biology of a growing population of Spanish imperial eagles Aquila adalberti across most of its breeding range. The objective of this study was to analyse the effects of age, supplemental feeding and rabbit haemorrhagic disease (RHD) on several breeding parameters of this population of eagles. Average clutch size was 2.2 eggs per clutch, and the average incubation time was 41.7 days per clutch. Fledging occurred an average of 76.8 days after hatching, the length of the fledgling period was not correlated to clutch size. The annual average percentage of pairs laying eggs was 88%. A significant reduction in the percentage of pairs laying eggs in the period 1992-1994 (from 91 to 81%) coincided with most of the eagles' territories being affected by the rabbit epizootic disease, RHD, which reduced their food supply significantly.Average productivity was 1.23 chicks per monitored territory, average breeding success was 1.40 chicks in a clutch per territory and the average fledging rate was 1.69 chicks per territory with hatching success. The main known causes of breeding failure during incubation were nest collapse and human disturbance. During chick-rearing, total or partial chick losses were mainly caused by siblicide, disease, malnutrition or nest collapse. In 26.2% of the 1372 monitored breeding attempts, at least one of the breeding birds was a subadult (the male in 56.1% of the cases, the female in 15.5% and both sexes in 28.4% of cases). In cases of mixed-aged pairs (n = 205), 70.7% were the result of a substitution, and 29.3% were the result of the forming of a new pair. Egg laying took place significantly earlier and breeding success was higher in territories occupied by adults than in those occupied by subadults. Breeding parameters were higher in high-quality (rabbit-rich) territories than in low-quality (rabbit-poor) territories, but only for those territories occupied by adults. The values obtained in the territories occupied by adults were only significantly higher than in those of the subadults in highquality territories. Age and territory quality thus simultaneously affected reproductive output.

Assessing the causes of breeding failure among the rough-legged buzzard (Buteo lagopus) during the nestling period

“…For instance, hatching asynchrony leads to asynchrony in chick development (Lack 1947), older chicks become stronger than younger ones and can kill them or scavenge them after their death. Avian siblicide is defined as ''juvenile mortality resulting from the overt aggression of siblings'' (Mock et al 1990: 236) and can be divided into two main groups: obligate, when more than 90% of last hatched chicks are killed by their siblings, and facultative, when incidence of siblicide varies with environmental circumstances (Simmons 1988;Mock et al 1990;Margalida et al 2004;Margalida et al 2007). Facultative siblicide usually occurs when feeding of the chicks is insufficient, for instance, due to poor territory quality, low breeding experience of the breeding pair or scarcity of resources (Wiehn & Korpimä ki 1997;Estes et al 1999;Gonzá lez et al 2006).…”

Space Use of Adult Spanish Imperial EaglesAquila adalberti

“…The breeding population is sedentary and monogamous (although see Gon zález et al 2006a) and egg-laying takes place be tween March and April (Margalida et al 2007b). Fledging takes place in June and July and, after around two months in the parental territory, the young begin to disperse (Margalida et al 2007b).…”

“…The breeding population is sedentary and monogamous (although see Gon zález et al 2006a) and egg-laying takes place be tween March and April (Margalida et al 2007b). Fledging takes place in June and July and, after around two months in the parental territory, the young begin to disperse (Margalida et al 2007b). Although juvenile dispersal has been studied (González et al 1989, Ferrer 1993, González et al 2006b), information regarding range and territorial behaviour in breeding individuals is anecdotal and unquantified (Valverde 1960, Meyburg 1975, Ferrer 1993.…”

Status and habitat changes in the endangered Spanish Imperial Eagle Aquila adalberti population during 1974–2004: implications for its recovery

“…This reduction negatively affected the population breeding parameters of this eagle (González et al 2006a, Margalida et al 2007. The fact that the new territories established after 1989 appear mainly in low quality habitats on the periphery of the breeding nuclei, could also explain the increase in the number of low quality territories.…”