Breeding dates and reproductive performance

Much of the evidence for the effects of climate change on birds comes from studies of the timing of breeding. Many species of birds start to lay eggs earlier in years with warmer temperature, and approximately 60% of studies have shown long-term advances in laying dates consistent with global warming. Nevertheless, the magnitude of the responses differs among species and locations in ways that we do not yet understand. Some of the variation is probably due to differences in: (1) local temperature changes, (2) diet, (3) body size, (4) life history (migratory or resident; number of broods per season) and (5) the time scales over which species acquire resources for breeding. Although earlier laying of clutches is often associated with larger clutch sizes and greater production of young, the effects of earlier laying on reproductive performance are less clear. In the future, we need to establish large-scale collaborative studies to monitor the effects of climate change over large areas and to determine how climate change affects the reproductive performance of species throughout their ranges.