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

Global climate change has altered the timing of seasonal events (i.e., phenology) for a diverse range of biota.

Oversummering juvenile and adult Semipalmated sandpipers in Perú gain enough survival to compensate for foregone breeding opportunity.

Background: Age at maturity and the timing of first breeding are important life history traits. Most small shorebird species mature and breed as 'yearlings', but have lower reproductive success than adults. In some species, yearlings may defer northward migration and remain in non-breeding regions ('oversummering') until they reach 2 years of age.