It is clear that discards from commercial fisheries are a key food resource for many seabird species around the world. But predicting the response of seabird communities to changes in discard rates is problematic and requires historical data to elucidate the confounding effects of other, more 'natural' ecological processes. In the North Sea, declining stocks, changes in technical measures, changes in population structure and the establishment of a recovery programme for cod (Gadus morhua) will alter the amount of fish discarded. This region also supports internationally important populations of seabirds, some of which feed extensively, but facultatively, on discards, in particular on undersized haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus). Here we use long-term data sets from the northern North Sea to show that there is a direct link between discard availability and discard use by a generalist predator and scavenger--the great skua (Stercorarius skua). Reduced rates of discarding, particularly when coupled with reduced availability of small shoaling pelagic fish such as sandeel (Ammodytes marinus), result in an increase in predation by great skuas on other birds. This switching of prey by a facultative scavenger presents a potentially serious threat to some seabird communities.

Adaptive sex-ratio theory predicts that parents should overproduce the more bene¢cial o¡spring sex. Based on a recent experimental study of lesser black-backed gulls, we tested this hypothesis with the great skua, Catharacta skua, a bird species closely related to gulls but where females are the larger sex. When in poor body condition, the gulls overproduced daughters, the smaller and more viable sex under those circumstances. To discriminate between a mandatory physiological overproduction of female (i.e. nonmale) eggs versus the overproduction of the smaller and presumably more viable sex, we conducted an egg-removal experiment with the great skua. Since the males are smaller, larger size and being male are separated. Through egg removal we induced females to increase egg production e¡ort. Eggs were sexed using a DNA-based technique. Manipulated pairs produced a signi¢cant male bias at the end of the extended laying sequence, while the sex ratio in the control group did not di¡er from unity. Our results present an example of facultative sex-ratio manipulation and support the hypothesis that in sexually dimorphic birds parents overproduce the smaller sex under adverse conditions.

Effects of experimentally increased egg production on female body condition and laying dates in the great skua Stercorarius skua . Á/ J. Avian Biol. 35: 501 Á/514.We investigated the effects of increased egg production on body condition as well as on measures of reproductive performance in great skuas, Stercorarius skua , over two subsequent years. We experimentally increased egg production from the normal two to six eggs. Six eggs might also be produced under natural circumstances after repeated clutch loss. After the production of the last egg we measured: (i) body mass, (ii) pectoral muscle, and (iii) haematocrit, total red blood cell count and mean corpuscular volume, as indicators of body condition. We took the same measurements of control females who had produced the normal clutch of two eggs. The measurements were repeated one year after the manipulation, and survival, laying dates, clutch sizes and hatching success were recorded for up to three consecutive years. After producing six eggs, females were lighter, had smaller pectoral muscles and lower haematological values than control females. Hatching success of eggs was significantly reduced. Even one year after the experiment there were still differences in body condition. Annual survival was not affected by the manipulations, although there was an indication that survival costs depended on whether chicks were raised after the increased egg production. While pair bonds and egg sizes were not affected in the postexperimental year, females started breeding significantly later than in the previous year. Two years after the experiment laying dates had advanced again and were not different from those of control females. This pattern of maintaining survival and egg sizes, but delaying breeding in the post-experimental year was found for two independent groups of females which had both been subjected to increased egg production. These results present evidence that increased egg production can have longterm effects on female body condition and aspects of reproduction. However, although present, the costs of extra eggs appear to have been relatively small in the great skua in comparison to the two other bird species for which inter-annual effects have been reported.

The larger sex is often more vulnerable, in terms of development and survival, to poor conditions during early life. Differential vulnerability has implications for parental investment strategies such as sex ratio theory. When males are larger, it is not possible to separate the effects of larger size per se and other aspects of the male phenotype on vulnerability. Furthermore, offspring competition might favor the larger sex and thereby mask intrinsic, size-related effects. We studied sex-specific mortality in a bird species with reversed size dimorphism, the great skua Stercorarius skua, under natural and experimentally created poor conditions. Small eggs from extended laying sequences were used to create poor early conditions for the offspring, which were raised as singletons. Daughters had a lower survival in all treatment groups. Survival in natural broods was additionally affected by hatch date and position. Hatch weight was not different for sons and daughters but was lower in experimental than in natural nests. In natural nests, daughters fledged 10% heavier than sons, but in experimental nests, they did not reach a higher mass. The average survival difference between sons and daughters was not increased in experimental broods. However, hatch weight had a strong sex-specific effect. Very light females never survived, and survival probability of daughters increased with increasing hatch weight. By contrast, survival of sons over the same range of hatch weights was not related to weight. These findings support the hypothesis that larger (final) size per se is related to sex-specific offspring vulnerability during early life.

This chapter uses recent experimental and observational studies of birds to explore patterns of sex-specific offspring vulnerability (increased mortality and reduced fledging mass under poor conditions) in relation to sexual size dimorphism (SSD). The results show size-dependent modulation of male fledgling mass but size-independent mass reduction in females. Overall, growth is more phenotypically plastic in males than in females. Comparisons of fledging mass reached in ‘good’ and ‘poor’ environments suggest that having to grow large is mainly disadvantageous when coupled with the male phenotype. Differences in environmental sensitivity between the two sexes during ontogeny, either in the form of increased mortality or reduced body size, will tend to reduce dimorphism during development, affecting adult SSD. These results suggest that environmental conditions during ontogeny contribute significantly to variation in SSD within bird species, particularly when comparisons are made among environments or between generations.

No abstract

Adoption of unrelated offspring by successful breeders is one form of brood mixing and alloparental care that is widespread among geese and other waterfowl. Biparental care and long‐lasting family bonds in geese are likely to affect the costs and benefits of adoption. Most hypotheses that have been proposed to explain this behaviour assume that the separation of the gosling from its original family is accidental, and that adoption forms the ‘best of a bad job’ solution. For the gosling, adoption is therefore thought to be the only, and thus adaptive, option. For parents, some hypotheses assume that there are costs of adoption (intergeneration conflict), while others assume that there are benefits (mutually beneficial). The few studies of adoption in wild goose populations indicate cost‐neutrality or that there are small benefits to parents of adopting young. This agrees with studies of brood size, which suggest that large families provide benefits for goslings and parents alike. By contrast, most observed adoption attempts involve parental aggression against the lone gosling. However, incidental observations are likely to be biased towards adoptions that involve conspicuous behaviour, such as aggression, and might overlook inconspicuous adoptions. Studies of individually marked goslings are needed to identify the background of the adoption goslings in order to identify whether in geese, as in some larids and altricial species, adoption might be an active strategy of offspring to improve their fitness prospects. In addition, more experimental studies are needed to test predictions about the costs and benefits of large families in geese.

No abstract