Population decline in an avian aerial insectivore (Tachycineta bicolor) linked to climate change
tree swallow , climate change , insect availability , population decline , long term monitoring , avian aerial insectivore
Avian aerial insectivores, a taxonomically diverse guild of birds, are facing dire population declines. The primary commonality among these birds is that they forage on flying insects, suggesting that diet has exposed these birds to environmental challenges that cause their decline, but it has been unclear how. For most aerial insectivores the demographic data necessary to isolate the cause of decline are lacking. However, using data from a tree swallow (Tachycineta bicolor) population that has been continuously monitored from 1975-2017, I investigated demographic and environmental causes of population decline. In my first chapter, I conducted a life stage simulation to determine which demographic transitions had the most influence on population growth rates, finding that juvenile and adult survival overwinter fledging success had the potential to influence population growth. In my second chapter, I found that both juvenile survival and fledging success declined concurrently with the overall population decline. Poor fledging success was associated with increased predation and rainy, cold weather during nestling development. When raining or cold, the flying insects nestlings rely on are inactive, likely causing temporary food shortages. Low juvenile survival overwinter was linked to poor weather conditions during the post-fledging period and perhaps to conditions on the wintering grounds. Finally, in my third chapter I show that the body mass of older nestlings that are approaching fledging has declined over time. In 2017, nestlings were lighter for their age after rainy weather, suggesting that increasingly poor growth could be explained by more spring rain. As a consequence of climate change, rainfall during nestling development had increased 9.3±0.3 mm/decade, explaining poor nestling growth, fledging success, and potentially juvenile survival. Overall, my findings show that declining nestling and juvenile survival may be driven by increasingly rainy weather, which may in turn cause decline in this tree swallow population and avian aerial insectivores more generally. Therefore, I suggest that tree swallows and other avian aerial insectivores be added to the growing list of species threatened by climate change.