Age-dependent diet choice in an avian top predator

Abstract

Age-dependent breeding performance is arguably one of the best-documented phenomena in ornithology. The existence of age-related trends has major implications for life-history theory, but the proximate reasons for these patterns remain poorly understood. It has been proposed that poor breeding performance of young individuals might reflect lack of foraging skills. We investigated this possibility in a medium-sized, powerful raptor-the northern goshawk Accipiter gentilis. Male goshawks are responsible for providing their females and their offspring with food. We hypothesized that young males may generally show poor breeding performance or even delay breeding, because they lack the experience to hunt efficiently-especially, their principal avian prey, the feral pigeon Columba livia. Our study exploited a rare 'natural experiment', the expansion phase of an urban population, where intraspecific interference was negligible and many young males bred successfully. This enabled us to examine the improvement of foraging skills in a larger sample of young individuals, and in more controlled conditions than usually possible. Using data from individually identified male breeders, we show that, consistent with our hypothesis, the proportion of pigeons in the diet increased significantly with male age, for at least the first three years of life. Other studies have shown a parallel increase in productivity, and a positive effect of a pigeon-rich diet on brood size and nestling condition, stressing the potential fitness relevance of this prey species for goshawks. Our results suggest a causal link between patterns of age-dependence in foraging ecology and reproductive performance. Furthermore, our study is, to our knowledge, the first demonstration that prey choice of breeders, which might reflect individual hunting skills, is age-dependent in a raptor.

Figure 1

The proportion of feral pigeons in the breeding-season diet of goshawk pairs in the city of Cologne, Germany, in relation to the age of the respective male breeders. (a) Brood-level raw data are plotted together with the best-fit line from a GLMM (‘year’ and ‘male identity’ fitted as random effects). The inset shows the same relationship when the analysis was restricted to prey samples from a time period when only male hawks hunted (data pooled across broods, because of small sample sizes). (b) Raw data (±s.e.) for three male age classes are shown for three different time periods of the colonization event (‘early’=1989–91, ‘middle’=1992–94, ‘late’=1995–97); one datum was randomly picked per male and time block (i.e. a maximum of three data points per male was used to calculate the values for this illustration, but only one datum per male was used in the statistical analyses, see panel c and §3). (c) Estimates of coefficients (±s.e.) from a resampling approach (mean from 100 runs) are shown, where one datum was randomly picked from each of the 41 individual males (i.e. exactly one data point per male). Both (b) and (c) demonstrate that the observation of age-dependent diet (a) is unlikely to be an artefact of temporal changes in environmental pigeon availability. For statistics and methodological details, see text.

Figure 2

Diet trajectories for individual breeding male goshawks (a) (individuals that bred for at least 2 years; data points joined for samples of 2 years, and linear regressions fitted for samples of 3 or more years); (b) correlation between diet composition of first-time breeding males (proportion of feral pigeons in diet of first breeding attempt) and longevity (number of years recorded breeding). Panel (a) suggests improvement of individual foraging competence (increases shown in black, decreases in grey), but on the basis of (b), population-level changes in phenotype frequencies cannot be ruled out.

Figure 3

Reproduction in relation to (a) diet composition and (b) age in goshawks. (a) Brood size increases significantly with the proportion of pigeons in the diet of pairs (a) (n=37 broods; linear regression: p<0.01; redrawn from Krüger & Stefener 1996). (b) Age-dependent reproduction in goshawks mirrors age-dependent diet choice (cf. figure 1). The relationship (mean±s.e.) is shown for (b(i)) male breeders and (b(ii)) for female breeders (male age: white bars, n=553 broods, Nielsen & Drachmann 2003; hatched bars, n=28 broods, Kenward et al. 1999; female age: circles, n=929 broods, Nielsen & Drachmann 2003; squares, n=919 broods, Risch et al. 2004; triangles, n=186 broods, Krüger 2005).