Data from: Inter-annual variability and long-term trends in breeding success in a declining population of migratory swans

Population declines among migratory Arctic-breeding birds are a growing
concern for conservationists. To inform the conservation of these
declining populations, we need to understand how demographic rates such as
breeding success are influenced by combinations of extrinsic and intrinsic
factors. In this study we examined inter-annual variation and long-term
trends in two aspects of the breeding success of a migratory herbivore,
the Bewick's swan Cygnus columbianus bewickii, which is currently
undergoing a population decline: 1) the percentage of young within the
wintering population and 2) mean brood size. We used an
information-theoretic approach to test how these two measures of
productivity were influenced over a 26 yr period by 12 potential
explanatory variables, encompassing both environmental (e.g. temperature)
and intrinsic (e.g. pair-bond duration) factors. Swan productivity
exhibited sensitivity to both types of explanatory variable. Fewer young
were observed on the wintering grounds in years in which the breeding
period (May to September) was colder and predator (Arctic fox) abundance
was higher. The percentage of young within the wintering population also
showed negative density-dependence. Inter-annual variance in mean swan
brood size was best explained by a model comprised of the negative degree
days during the swan breeding period, mean pair-bond duration of all
paired swans (i.e. mean pair duration), and an interaction between these
two variables. In particular, mean pair duration had a strong positive
effect on mean brood size. However, we found no long-term directional
trend in either measure of breeding success, despite the recent decline in
the NW European population. Our results highlight that inter-annual
variability in breeding success is sensitive to the combined effects of
both intrinsic and extrinsic factors.