S20.Summary: Reproductive strategies in marine birds

Marine birds characteristically show high adult survival, low annual reproductive output and delayed maturity. Life history theory leads us to predict that their reproductive decisions will favour high adult survival at the expense of annual reproductive success. These life history traits make them particularly suitable for studies of the adaptive significance of particular reproductive strategies, since it is possible to examine variation between and within individuals, populations and species.

Conditions in the marine environment are highly variable, generating both spatial and temporal variation in resource availability. There is high intra-specific variation in the capacity of individual birds to obtain these resources and individuals generally breed several times in differing environmental circumstances. In addition to differences in individual quality, an important proximate factor likely to influence reproductive decisions at the individual level is body condition, which will vary considerably in relation to environmental circumstances. Thus, the optimal strategy for particular individuals will be state-dependent, determined by a combination of inherent quality and current condition of both the individual itself and, of course, that of its mate.

There are a number of different approaches to the study of the adaptive significance reproductive decisions, and these have been illustrated in this symposium. One disadvantage in studying seabirds is their very long lifespan, which means that, in order to capture the temporal variation in performance, it is necessary to conduct long-term studies over many years. This requires longevity and persistence on the part of the observers, and a degree of confidence in the value of such work that is not always forthcoming from funding bodies. The paper presented here by Wooller and Bradley illustrates the considerable value of long term studies of seabirds in enabling us to both quantify variation, and elucidate key variables affecting performance. Experimental and comparative studies are also very important in teasing apart confounding variables involved in a particular issue, as is illustrated in the paper by Seddon on brood reduction in penguins. The importance of parental body condition in determining the provisioning pattern of parents rearing chicks is very clearly shown in the paper by Weimerskirch, while the effect of parental condition on the subsequent recruitment of young to breeding colonies is examined in detail by Becker.

Reproductive decisions have important consequences for the demography of seabird populations, making them particularly sensitive to changes in the adult mortality rate. However, in practice, since seabirds have been selected to make decisions that do not jeopardise their survival, adult survival is generally high and relatively stable. From a conservation viewpoint however, anthropogenic effects that cause high adult mortality rate do pose a particular threat to seabird populations.

Overall, a consensus view from this symposium is that understanding the causes and consequences of variation in adult condition represents a particular challenge for future studies of breeding seabirds at the individual and population level. Experimental work, in which condition is manipulated, has a important contribution to make in testing hypotheses generated from detailed quantitative studies of both the birds and the environmental conditions that they experience.