Effects of extreme weather on two sympatric Australian passerine bird species

Abstract

Despite abundant evidence that natural populations are responding to climate change, there are few demonstrations of how extreme climatic events (ECEs) affect fitness. Climate warming increases adverse effects of exposure to high temperatures, but also reduces exposure to cold ECEs. Here, we investigate variation in survival associated with severity of summer and winter conditions, and whether survival is better predicted by ECEs than mean temperatures using data from two coexisting bird species monitored over 37 years in southwestern Australia, red-winged fairy-wrens, Malurus elegans and white-browed scrubwrens, Sericornis frontalis. Changes in survival were associated with temperature extremes more strongly than average temperatures. In scrubwrens, winter ECEs were associated with survival within the same season. In both species, survival was associated with body size, and there was evidence that size-dependent mortality was mediated by carry-over effects of climate in the previous season. For fairy-wrens, mean body size declined over time but this could not be explained by size-dependent mortality as the effects of body size on survival were consistently positive. Our study demonstrates how ECEs can have individual-level effects on survival that are not reflected in long-term morphological change, and the same climatic conditions can affect similar-sized, coexisting species in different ways.