UNDERSTANDING THE DRIVERS OF POPULATION DECLINES FOR SWALLOWS (FAMILY: HIRUNDINIDAE) THROUGHOUT THE ANNUAL CYCLE

Abstract

Swallows and other aerial insectivores are experiencing steep population declines, potentially as a result of decreased insect availability during breeding and poor non-breeding conditions. To determine the likely drivers of decline for Bank Riparia riparia, Barn Hirundo rustica, Cliff Petrochelidon pyrrhonoto and Tree Swallows Tachycineta bicolor and Purple Martins Progne subis and whether they were common to multiple species, I: 1) examined the relationships between insect abundance and swallow breeding success (2014-2015); 2) compared breeding phenology and performance before (1962-1972) and after (2006-2016) the onset of population declines; 3) examined relationships between non-breeding conditions and potential carry-over effects (2014-2016); 4) identified winter locations; and 5) reviewed the effect of several threats on adult survival. Insect abundance was not related to Barn, Cliff and Tree Swallow nestling survival or mass suggesting that it did not limit breeding success. Between 1962-1972 and 2006-2016, I found that Barn, Cliff and Tree Swallows bred 8-10 days earlier and had unchanged or higher performance. In contrast, Bank Swallows did not breed earlier and had lower performance. Poor non-breeding conditions, particularly low rainfall, resulted in carry-over effects during breeding (i.e., lower mass, later breeding or lower performance) for Barn and Cliff Swallows; these conditions were related to higher mass, but later breeding and lower performance for Bank Swallows. Stable isotope and geolocator results indicated that Bank, Barn and Cliff Swallows likely winter throughout Brazil, Bolivia, Paraguay, Uruguay and Argentina. While little information is available on relationships between threats and adult survival, poor weather is related to lower survival for all species except Purple Martins. Low insect abundance during the breeding season is likely not contributing to population declines for Barn, Cliff and Tree Swallows, but, for Bank Swallows, declines may be partly due to a mis-timing between food availability and breeding. Also, for Bank, Barn and Cliff Swallows, poor non-breeding conditions are associated with carry-over effects on breeding, including lower success. Poor non-breeding conditions may also contribute to population declines through lower adult survival. While there are some similarities in the response of many species to different potential drivers, Bank Swallows often differed in their response.