A framework for reproductive outcomes of phenological match-mismatch in migratory breeders tested on a declining species, caribou.

Long-distance migrants with endogenously timed reproduction may be especially vulnerable to phenological mismatch on summer ranges where offspring are produced and provisioned. In such species, departure timing from winter ranges, and consequently breeding timing on summer ranges, is cued primarily by photoperiod, while local temperature cues the timing of resource availability on summer ranges. Hence, studies of climate change-driven mismatch have focused largely on one aspect of resource phenology: the timing of resource availability on summer ranges. We present a framework for expected reproductive outcomes in long-distance migrants that integrates both seasonal timing and rate of increase in resource availability on summer ranges. The framework predicts that potential disadvantages of earlier resource availability relative to resource demand may be buffered by slow rates of increase in seasonal resource availability. Conversely, potential advantages of later resource availability typical of historically average timing that coincides with the timing of resource demand may be unrealized if resource availability increases too rapidly. We test the framework using 21 years of data on offspring production by tundra caribou, a long-distance migratory breeder whose global abundance is declining, whose timing of reproduction is endogenously entrained, and for whom resource availability on summer ranges may advance with local warming. Agreement with empirical outcomes for caribou, in particular, 3-fold greater offspring production in later springs with slow vegetation green-up compared with later springs with rapid green-up, suggests broader utility of this framework for predicting climate change consequences for phenological match-mismatch in other long-distance migratory breeders.