Weather-mediated natural selection on arrival time in cliff swallows ( Petrochelidon pyrrhonota )

Abstract

An unusually long period of cold weather in May 1996 caused extensive mortality among insectivo- rous cliff swallows (Petrochelidon pyrrhonota) in the northern and central Great Plains. We analyzed how via- bility selection affected spring arrival time in a migratory Nebraska population by comparing capture histories of survivors with those of birds known to have died and by documenting how arrival time changed in the year fol- lowing the selection event. Surviving birds had signifi- cantly later first-capture dates (an index of arrival time) in the years prior to selection than those that died; a sig- nificant selection differential suggested directional selec- tion for birds that arrived later. Colony sites were occu- pied significantly later following the selection event, and the distribution of first-capture dates in the season after selection was significantly shifted toward later arrivals. Offspring of the survivors tended to arrive later than birds of the same age prior to the selection event. While major weather-caused mortality events of this magnitude are rare in the study area, spells of cold weather severe enough to cause limited mortality are frequent in April and early May. At least 25 probable mortality events of varying severity were identified in the last 50 years based on climatological data. Periodic weather-mediated selection against early arrival constrains the cliff swal- low's breeding season and may partly prevent directional selection for earlier nesting.