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

An unusually long period of cold weather in May 1996 caused extensive morta lity among insectivorous cliff swallows (Petrochelidon pyrrhonota) in the n orthern and central Great Plains. We analyzed how viability selection affec ted spring arrival time in a migratory Nebraska population by comparing cap ture histories of survivors with those of birds known to have died and by d ocumenting how arrival time changed in the year following the selection eve nt. Surviving birds had significantly later first-capture dates (an index o f arrival time) in the years prior to selection than those that died; a sig nificant selection differential suggested directional selection for birds t hat arrived later. Colony sites were occupied significantly later following the selection event, and the distribution of first-capture dates in the se ason after selection was significantly shifted toward later arrivals. Offsp ring of the survivors tended to arrive later than birds of the same age pri or to the selection event. While major weather-caused mortality events of t his magnitude are rare in the study area, spells of cold weather severe eno ugh to cause limited mortality are frequent in April and early May. At leas t 25 probable mortality events of varying severity were identified in the l ast 50 years based on climatological data. Periodic weather-mediated select ion against early arrival constrains the cliff swallow's breeding season an d may partly prevent directional selection for earlier nesting.