Why do avian responses to changes in Arctic green-up vary?

Global climate change has altered the timing of seasonal events for a diverse range of biota. Within and among species, however, phenological responses to climate change vary substantially. To better understand factors driving this variability in organismal responses, we evaluated changes in timing of nesting of eight Arctic-breeding shorebird species at 18 sites over a 23-year period. We used the Normalized Difference Vegetation Index as a proxy to determine the start of spring growing season (SOS) and quantified inter and intraspecific relationships between SOS and nest initiation dates as a measure of phenological responsiveness. We then tested whether (i) interspecific patterns in the relationship between SOS and nest initiation dates covaried with four species traits (migration distance, female body mass, expected female reproductive effort, seasonal timing of breeding), and (ii) intraspecific patterns showed spatial variation across the Arctic. Although no species completely tracked annual variation in SOS, phenological responses were strongest for Western Sandpipers, Pectoral Sandpipers, and Red Phalaropes. Migration distance was the strongest additional predictor of responsiveness, with longer-distance migrant species generally being more sensitive to SOS than species that migrate shorter distances. Semipalmated Sandpipers are a widely distributed species but adjustments in timing of nesting relative to changes in SOS did not vary across sites, suggesting that different breeding populations of this species were equally responsive to climate cues despite differing migration strategies. Our results unexpectedly show that some long-distance migrants are more sensitive to local environmental conditions, which may help them to adapt to climate change.