Timing to temperature: Egg‐laying dates respond to temperature and are under stronger selection at northern latitudes

Abstract

AbstractGlobal climate change produces spatially variable patterns of environmental change. This could put migratory species at risk as the synchrony between migration timing and suitable breeding conditions could become mismatched. For migratory birds, whether the timing of egg laying is a plastic trait that can vary in response to environmental change has been sparsely studied across regions and systems and thus remains poorly known. We investigated the effects of temperature variability and climate warming on the breeding phenology of purple martins (Progne subis), a long‐distance migratory songbird, using a 20‐yr data set comprised of 28,165 records of nest timing and fledgling success spanning the entire breeding range (25–54° N). We discovered that purple martins lay eggs earlier in warmer springs and fledge more young when they lay earlier. After controlling for spatial patterns in the data with Moran's eigenvector maps, we found that selection favored earlier breeding in most years, particularly at more northern latitudes. However, selection pressure for earlier breeding did not increase over the 20‐yr period, perhaps owing to high variability in temperature across years. Our results therefore demonstrate plasticity in the timing of egg laying in response to temperature variation and climate change over 20 yr across the range of this widely distributed, long‐distance migrant. Whether these plastic responses are common or sufficiently matched to climate change among other declining migratory songbird species should be further investigated.