Long-term climate impacts on avian reproductive phenology in Pennsylvania
Climate change has already influenced the annual cycles of migratory bird species worldwide, as is evidenced by trends in advanced spring arrival and shifts in the timing and duration of breeding for many species. Although changes in migration phenology are well-studied, we still lack information on how many species are responding to climate change during the breeding season, particularly in the long-term. Our objective was to test whether dynamic climate variables (e.g., spring temperature, precipitation, and regional climate indices) affect timing of passerine reproduction at a constant effort mist-netting station in Pennsylvania. Using five decades of banding data we performed stepwise multiple regressions to examine which climate variables best explained yearly variation in indicators of reproductive timing and productivity (hatching of juveniles, appearance and duration of brood patch and gravidity, and juvenile:adult ratio). Additionally, we explored the influence of life history attributes on these trends. We predicted earlier nesting across all species in warmer springs indicated by earlier appearance of breeding condition in adult females, earlier appearance of juvenile birds (10% quantile of young within the breeding window), and a greater juvenile to adult ratio for multi-brooded species.
Several lines of evidence point to sensitivity of the reproductive phenology of breeding passerines to changing climate. Regardless of broodedness, migration distance, or habitat guild, we found that 11 of 23 species analyzed hatched young earlier and eight of 19 had earlier appearance of female breeding condition over time. Warm springs were associated with earlier captures of young for nine species, but this timing was not influenced by precipitation or regional climate indices. Appearance of brood patch/gravidity was earlier in warmer and drier springs for the majority of species analyzed, and four species of 19 exhibited extended breeding season length with increasing spring temperatures. In general, mean temperature and total spring/summer precipitation were positively related to productivity (for six and nine species, respectively, of 22). More bird species are expected to advance their laying dates under future warming scenarios, potentially benefitting some multi-brooded species. However, fitness consequences of changes in breeding phenology are complicated by potential mismatches in timing of resource abundance and resource reduction with warming temperatures. Further investigation is needed to understand the consequences of future climate change on population dynamics (i.e., season-specific survival and fecundity) to understand which species will best be able to adapt.