Migratory aerial insectivores have experienced significant population declines in North America over the last 30 years. Here, we test whether the reproductive success of Tachycineta bicolor (Tree Swallow) has been negatively impacted by global change, with a focus on precipitation patterns. In northeast North America – the area in which aerial insectivores are declining most dramatically – there has been a 67% increase in very heavy precipitation events and a 10% increase in annual precipitation over the last 60 years, and precipitation is predicted to continue rising. Rain events can prevent flight of insects, limiting foraging opportunities for aerial insectivores. Thus, more precipitation events can lead to a reduction in foraging and less food for adults and their young. However, the influence of increased precipitation during the breeding season on the reproductive success of aerial insectivores remains understudied. Using a 32-year (1987-2018) dataset of reproductive success and phenology of T. bicolor from a conservation area in South Natick, Massachusetts, we investigate reproductive timing and success of T. bicolor in response to a changing climate. We also investigate management strategies that may offset some of the negative effects of global change on T. bicolor breeding populations.
Results/Conclusions
We recorded significant declines in T. bicolor reproductive success, and found that these declines are associated with climate change. Over a period of 30 years, clutch sizes have declined by 0.5 eggs per nest. High early spring precipitation delays reproductive phenology, with T. bicolor laying eggs 0.4 days later per cm of rain. Further, clutches laid later in the season are smaller than those laid earlier. Fledging success is lowest in years with heavy June precipitation, meaning fewer offspring enter the potential breeding pool in wet years. Our observed patterns of delayed reproduction in rainy springs and smaller late season clutch sizes may be compounded in the future as spring precipitation continues to rise, and lead to further reductions in reproductive success.
We found significant variation in reproductive success within the conservation area. Clutches in nests near a water body are initiated earlier in the year than those in dry field habitats, which may offset the observed phenological delay associated with high spring precipitation. We also see higher fledging success in nest sites near water, even in years with high June precipitation. These results will directly inform management decisions about habitat provisioning in Mass Audubon Wildlife Sanctuaries.