Showing papers by "Ruedi G. Nager published in 2000"

Prey selection and foraging performance of breeding Great Tits Parus major in relation to food availability

Abstract: Naef-Daenzer, L., Naef-Daenzer, B. and Nager, R. G. 2000. Prey selection and foraging performance of breeding Great Tits Parus major in relation to food availability. ‐ J. Avian Biol. 31: 206‐214. We studied the nestling diet and the foraging performance of Great Tits in relation to prey abundance in the field. Numerous experimental studies present data on foraging decisions in captive Great Tits. Little is, however, known about prey selection in the field in relation to the food available and the consequences this has for the food delivery rate to nestlings. Since the foraging performance of the parents is one of the main determinants of fledging weight and juvenile survival, foraging behaviour is an important part of Great Tit reproduction. During the early breeding season up to 75% of the prey biomass delivered to the nestlings were spiders, which is in contrast with other studies. Only when caterpillars reached a size of 10‐12 mg (approximately the average size of the spiders caught at that time) did the Great Tits change their preferences and 80‐90% of the delivered prey masses were caterpillars, as reported by other authors. This ‘switching’ between prey occurred within a few days. It was not related to the changes in abundance but to size of caterpillars. The rate at which caterpillars were delivered to the nestlings (in mg:nestling:h) was strongly correlated with the caterpillar biomass available (in mg:m of branches) and nestling growth rate was significantly influenced by the mass of available caterpillars. The results provide evidence why perfect timing of breeding is so important for the Great Tit, and contribute to the understanding of the causal link between food supply, growth and breeding success.

Within-clutch trade-offs between the number and quality of eggs: experimental manipulations in gulls

Abstract: We experimentally increased the number of eggs laid by Lesser Black-backed Gulls and examined the relationship between egg number and egg quality. Egg quality was measured in terms of egg composition and the probability that an egg would give rise to a fledged chick. In measuring the latter, we removed the potentially confounding effects (1) between parental quality and egg quality and (2) between increased egg production and parental condition, using a cross-fostering protocol in which eggs were reared by control parents. Lesser Black-backed Gulls were capable of producing, on average, almost three times the normal clutch of three eggs. Although egg mass did not fall below that of the last laid egg in normal clutches, as the clutch extended beyond three, experimental eggs contained relatively less lipid and relatively more water. Overall, the percentage of eggs that gave rise to fledged chicks (when reared singly by a foster parent) declined significantly with position in the experimental laying sequence; about two-thirds of the eggs laid at the beginning of the laying sequence gave rise to fledged chicks, whereas only about one-third of the last laid eggs did so. This was not due to any seasonal change in the capacity of foster parents to rear the chicks, and neither hatching nor fledging success of the eggs was related to their fresh mass. Thus, as more eggs are laid, important changes in egg composition occur that have, in themselves, substantial effects on offspring survival. This study provides clear evidence of a trade-off between egg number and egg quality.

Parental condition, brood sex ratio and differential young survival: an experimental study in gulls (Larus fuscus)

Abstract: Empirical evidence is growing that the offspring sex ratio in birds can be biased in relation to the body condition of parents during breeding. The sex ratio bias may come about because (1) the actual production of the two sexes may be skewed and/or (2) there may be a sex bias in early nestling mortality contingent on parental condition. By manipulating parental condition and giving them a control brood to rear, thereby eliminating effects operating via the eggs, we examined the extent to which parental condition influences the post-hatching survival of male and female lesser black-backed gulls, Larus fuscus. We found that the pre-fledging survival of male chicks was strongly reduced in all-male broods reared by parents in poor condition. Pre-fledging survival of female chicks was, however, unaffected by parental condition or brood sex composition. Thus, independently of any production biases, sex differences in nestling mortality alone can bias the offspring sex ratio at fledging in relation to the prevailing rearing conditions. In other studies on gulls we have, however, also shown that females in poor condition at laying preferentially produce female eggs. Clearly a bias in fledging sex ratio can occur within the same species due to a combination of differential production and differential post-laying mortality; the latter can involve a differential effect of poor egg quality on male and female offspring, differential effects of brood sex composition on their survival and a difference in the capacity of parents to rear males and females. All of these processes need to be taken into account in attempting to understand offspring sex ratios.