Showing papers by "Ruedi G. Nager published in 2005"

Food availability affects adult survival as well as breeding success of parasitic jaegers

Abstract: Long-lived species are expected to reduce their current reproductive effort in adverse conditions in order not to jeopardize their future reproduction. Parasitic Jaegers (Stercorarius parasiticus) breeding in Shetland, United Kingdom, have declined substantially in numbers over the last 15 years, although the causes of this decline remain unclear. By carrying out a supplementary feeding experiment during the chick-rearing period, we tested the hypothesis that food availability has contributed to this decline. Data were collected on adult territory attendance, chick growth rates, and breeding success during the experimental season, and adult return rates, body condition, and laying dates during the following season. Differences in carbon and nitrogen isotope ratios in the natural and supplementary food, analyzed in the chicks' feathers, showed that chicks received one-fifth of their protein requirements from the supplementary food, but this amount varied between broods. Compared with controls, supplemented pairs showed higher attendance on the territory, improved breeding success, and higher return rate. This emphasizes that adult return rate is more sensitive to food availability than has been assumed previously. Control birds may have maintained too high a current reproductive effort at the expense of future reproduction, perhaps due to the unpredictability of their main food source, sandeels, which seemed abundant early in the season but scarce during chick rearing. This study suggests that the consequences of poor food availability are shared between offspring and parents in this species.

Intra-specific interactions influence egg composition in the lesser black-backed gull (Larus fuscus)

Abstract: Egg composition, which is under maternal control, can have a profound effect on offspring fitness. The presence of maternal testosterone and carotenoids in avian egg yolk, for example, is thought to enhance the development and competitive ability of the offspring and protect the hatching and growing chick against oxidative stress. Egg quality often differs between females and such variation can be due to differences in maternal social environment, e.g. breeding density. However, this is confounded by the possibility that the quality of individuals breeding in high- or low-density areas may vary. We tested if maternal social environment influences egg composition in a colonial seabird, the lesser black-backed gull (Larus fuscus). To control for confounding effects of female quality, we experimentally manipulated maternal social environment during egg formation. We increased the frequency of intra-specific interactions (i.e. aggressive encounters with conspecifics other than nest mates) in which the females were involved, by placing an elevated platform in their territory. Females that took part in more intra-specific interactions produced a heavier last egg, but the yolk testosterone concentration in eggs laid by control and experimental females did not differ. Differences in yolk testosterone concentration in relation to embryo sex were found neither in the control nor in the experimental group. In contrast, within the control group, eggs with a male embryo contained more carotenoids than eggs with a female embryo. Moreover, experimental females that had been involved in more intra-specific interactions produced female eggs with higher carotenoid levels compared to female eggs of control birds. An experimental increase in carotenoid levels was not observed in eggs containing a male embryo. Our results suggest that intra-specific interactions experienced by female birds during egg formation can influence conditions for embryonic development.

Sex allocation in response to paternal attractiveness in the zebra finch

Abstract: Females mated to attractive males are predicted to produce male-biased broods. Previous studies on zebra finches, Taeniopygia guttata, in which colored leg rings were used to alter male attractiveness, support this hypothesis. However, because molecular sexing techniques were not available, it was not known when during development this bias arose. Also, because both attractive (red-ringed) and unattractive (green-ringed) males were within the same aviary, assortative mating between treatments may have confounded the results. Using two different experimental designs, we tested whether the sex ratio of zebra finch eggs and chicks differed in response to paternal ring color whilst controlling for assortative mating between treatments. In the aviary experiment, birds could interact socially, but all males in an aviary had the same leg ring color. In the cage experiment, each female was randomly assigned a red- or green-ringed mate, thus also eliminating assortative mating within treatments. Offspring were sexed based on plumage or using a molecular method. The sex ratio at laying did not differ between treatments in either the aviary (n e 313 eggs) or cage (n e 151 eggs) experiments, suggesting that female zebra finches do not manipulate the primary sex ratio in response to their mate's ring color. However, in the cage experiment we found greater male embryonic mortality in the attractive group, which resulted in a female-biased sex ratio at sexual maturity, that is, in the opposite direction to that found in previous studies. Possible explanations for the disparity between our results and those of previous studies are considered. Copyright 2005.

Incubation effort in relation to male attractiveness in zebra finches Taeniopygia guttata

Abstract: Gorman, H. E., Arnold, K. E. and Nager R. G. 2005. Incubation effort in relation to male attractiveness in zebra finches Taeniopygia guttata . / J. Avian Biol. 36: 413/420. The division of labour in parental care between the two sexes varies between and within species. In birds, parents have been shown to invest more into egg production and nestling care when paired with an attractive rather than an unattractive mate, as predicted by the differential allocation hypothesis. Here we investigate variation in the female’s and male’s share of incubation behaviour, a vital, and costly, period of parental care during which the embryo is vulnerable to perturbations in developmental conditions. We manipulated the attractiveness of male zebra finches Taeniopygia guttata , using red or green leg-rings. To simulate their natural social environment we allowed them to breed in outdoor aviaries. All males within an aviary were given the same coloured ring to avoid ring-colour related assortative mating. Males within a colony, however, were still expected to show some variation in attractiveness with the earliest laying females possibly pairing with the most attractive males. Indeed we found that both factors played a role in explaining female incubation effort. Among females mated to red ringed males, earlier laying females contributed significantly more to incubation than late laying females, but no such pattern was found in females mated to green ringed males. Overall, there were no differences in the level of incubation provided by both parents between treatment groups, suggesting some compensation within the pair. Hatching success was correlated with a pair’s total incubation effort. These results suggest that variation in the division of parental care between the sexes is in agreement with both increased effort of females mated with attractive males, and females compensating for the reduced effort of attractive males seeking further mating opportunities. These two factors can act at the same time in natural populations and both should be considered when explaining variation in division of labour between the sexes.

Effects of incubation conditions and offspring sex on embryonic development and survival in the zebra finch (taeniopygia guttata)

Abstract: Suboptimal conditions during embryonic development can affect offspring fitness. Both egg quality and incubation behavior can affect hatching success, hatching mass, and subsequent offspring performance. These effects may differ between male and female offspring. We manipulated the prebreeding body condition of Zebra Finches (Taeniopygia guttata), using diets of different protein content. To separate possible effects on egg quality of parental body condition and incubation conditions, we did a cross-fostering experiment. We analyzed embryo survival and hatching mass with respect to body condition of the egg-laying parent, body condition of the incubating foster parent, and offspring sex. Embryos were not affected by the condition of the egg-laying parent. Eggs incubated by parents in better condition suffered less embryo mortality than those incubated by parents in poorer condition, but only when overall embryo mortality was low. Hatching mass was also affected by the incubating foster parent’s b...