Effects of Migration Distance on Shifting Migratory and Breeding Phenology in Waders.

Shifts in phenology are widely reported across taxa and, among migratory birds, advancing timing of breeding has occurred predominantly in short-distance migrants. Long-distance migrants might be less able to advance breeding if they arrive later and breed soon after arrival, but opportunities to quantify trends in phenology across species that experience similar breeding conditions but vary in migration distances are rare. Between 2007 and 2022, we recorded arrival and laying dates across lowland Iceland for nine wader species that vary in migration distances. Waders wintering closer to Iceland arrived ~6 weeks earlier than those wintering further away, yet laying dates differed by only ~1-2 weeks. Over this survey period, short-distance migrants advanced laying despite little or no advance in arrival, while long-distance species advanced both arrival and laying dates. The longer arrival-laying interval in species travelling shorter distances appears to allow earlier laying in warm springs, a flexibility less available to later-arriving species. Due to the benefits of breeding early in migratory systems, the opportunity of early nesting in warming springs could be contributing to divergent population trajectories of short- and long-distance migrants. Quantifying the phenology of nest and fledging success of species migrating over different distances will help to identify the costs of travelling further and arriving later during this period of rapid environmental change.