Age and territory-quality effects on fecundity in the Spanish Imperial Eagle (Aquila adalberti)

The Auk 120(1):180-186, 2003

ABSTRACT.-Over a period of 19 years, we studied 237 breeding attempts of Spanish Imperial Eagles (Aquila adalberti) in Donana National Park (southwestern Spain), including 29 pairs with at least one immature member, to investigate age-related effects on population fecundity. Without considering effect of territory quality, adult pairs were significantly more productive than immature pairs. Highly significant differences in breeding performance among territories were independent of age-classes of birds occupying them. Low-quality territories were more frequently occupied by immature pairs, whereas high-quality territories were occupied mostly or exclusively by adult birds. Therefore, age and territory quality appear to be interrelated. We found no effect on average population fecundity due to breeding by immature pairs. Received 13 November 2007, accepted 26 October 2002.

STUDIES OF age-related parental changes in reproductive success of many avian species have shown that older individuals nest earlier in the season, produce larger clutches, and have greater fledging success than do younger conspecific individuals (Newton 1979, Saether 1990, Wooller et al. 1990, Hamer and Furness 1991, Sydeman et al. 1991, Desroschers 1992, Hepp and Kennamer 1993, Forslund and Part 1995). Understanding how age influences breeding success is important to studies of population dynamics and to our understanding of the relationship between habitat and productivity.

The Spanish Imperial Eagle (Aquila adalberti) is the most endangered bird of prey in Europe and one of the rarest raptors in the world (Collar and Andrew 1988). The world population is estimated to be ~130 pairs (Ferrer 1993a). The Spanish Imperial Eagle population at Donana National Park consists of 15-16 pairs breeding at a high density (occupying 20,000 ha of available habitat inside the national park with a mean territory size of 1,200 ha; Ferrer 1993b). Typically, breeding pairs include two adults, but some pairs have one or two members that have not attained adult plumage (

In territorial birds, territory quality is likely to differ greatly among pairs (Newton 1979, 1991; Hogstedt 1980; Ferrer and Donazar 1996). Immature birds tend to occupy low-quality territories (Newton et al. 1981, Steenhof et al. 1983), so it is possible that habitat quality may generate age-related differences in reproductive success. The specific goal of this study was to investigate age-related differences in fecundity in the Spanish Imperial Eagle and its effects on the population of the Donana National Park, considering territory quality as a possible confounding effect.

METHODS

Study area.-The study was conducted in Donana National Park, southwestern Spain (37[degrees]N, 6[degrees]30'W) from 1976-1995. Three main habitats can be distinguished in the area: (1) Mediterranean scrubland, formed by Halimium spp., Cistus libanotis, Erica spp. with scattered cork oaks (Quercus suber), small stone pine (Pinus pinea) woods, and Eucalyptus spp. plantations; (2) marsh, mainly covered by Scirpus spp., flooded in winter and dry in summer; and (3) coastal sand dunes, with vegetation mainly made up of Ammophila arenaria, Corema album, and Juniperus phoenicia. The climate is Mediterranean with an Atlantic influence.

The species.-The Spanish Imperial Eagle is a large (2500-3500 g), sedentary and territorial bird of prey, with a low reproductive rate (0.75 chicks per pair per year), a four to five year immature phase, and a estimated longevity of 21-22 years (Ferrer and Calderon 1990). The Spanish Imperial Eagle may be divided into three easily distinguishable plumage classes: (1) juvenile, with a tawny-colored plumage that remains until the bird is 3 years old; (2) subadult, with dark patches over a tawny base, present in 4-5 year old birds; and (3) adult, predominantly dun-colored with characteristic white markings present in birds from the age of 5 years or older.

Paired birds are territorial. Territories are areas of exclusive use that arc vigorously defended throughout the year. In contrast, immature unpaired eagles are not territorial (Ferrer 1993b) and move between temporary settling areas, including the natal population area, throughout the dispersal period (Ferrer 1993c).

Data and statistical analyses.-We used data from the Donana archives for 1976-1995, a period during which the population remained stable (Ferrer and Donazar 1996) and visits to nests was made with an identical pattern of visitation to all nests in all years. Visits were performed by two or three people, one of whom climbed to the nest to record breeding stage and examine eggs or young. Nesting attempts were monitored three times during the breeding season (February-September): once at stages of egg laying, nestling, and fledgling (when the nestling reached at least 50 days of age, the age of banding). We considered laying date, number of eggs, number of nestlings, and number of chicks fledged as measures of reproductive performance. The entire national park area was surveyed at the beginning of each breeding season (January-February, during the courtship and nest-site selection period) to determine if pairs were present on territories. In summary, we knew population size, number of pairs that started reproduction, and productivity for 19 years.