Breeding phenology and performance for four swallows over 57 years: relationships with temperature and precipitation

Climate change can drive population declines for many species, often through changes to their food supply. These changes can involve a mis-timing between periods of high food demand and peak food availability, typically from advances in breeding phenology, and/or an overall reduction in food availability. Aerial insectivores, birds that feed on insects caught in flight, are experiencing steep population declines possibly because of shifts in the timing and/or abundance of aerial insects. We determined whether changes in breeding performance over time could account for declines in Bank Riparia riparia, Barn Hirundo rustica, Cliff Petrochelidon pyrrhonota, and Tree Tachycineta bicolor Swallows, and if so, whether changes were related to shifts in breeding phenology and/or climate change. We compared breeding performance and phenology in Maritime Canada before (1962-1972) and after (2006-2016) the onset of steep population declines during the mid-1980s, to determine whether breeding performance was reduced or phenology was advanced. Then, we modeled relationships between temperature, precipitation, breeding phenology, and performance for Barn and Tree Swallows, the only species with sufficient data, from 1960 to 2016, to determine whether phenology and performance were related to climatic conditions. Between the two time periods, we found significantly lower performance in Bank Swallows, higher performance in Barn and Tree Swallows, and unchanged performance in Cliff Swallows. We also found clutch initiation dates advanced by 8-10 d for all species except Bank Swallows. On the breeding grounds, warmer winter temperatures for Tree Swallows and less winter precipitation for Barn and Tree Swallows in a given year were associated with earlier breeding, and for Tree Swallows, changes in nestling survival. Otherwise, Barn and Tree Swallow breeding performance was unaffected by winter temperature and precipitation. Our results suggest that in this region poorer breeding performance could contribute to population declines for Bank Swallows but not for the other three species.