Divergent demographic responses of boreal-breeding ducks to growing season variability.

In seasonal environments climatic variability shapes the timing, duration, and magnitude of primary production with effects that may percolate upward through food chains. While the role of spring phenology and implications of trophic mismatches have been documented in some bird species, including waterfowl, there is little research on the role of growing season duration or overall productivity. Duck species breeding in the western boreal forest (WBF) of North America vary widely in their average timing and plasticity for breeding dates, which may set up differing sensitivity to spring phenology. In contrast, increases in growing season duration and productivity may positively impact species regardless of life history through the extension of the breeding season and bottom-up trophic enrichment. We tested these hypotheses using breeding population estimates for 8 species (or species groups) of ducks and normalized difference vegetation index to infer spring phenology, length of growing season, and growing season productivity for duck survey areas in the WBF, 1982-2019. Spring phenology had mixed effects on species' population growth rates, and effects were generally not consistent with mediation by average breeding dates or plasticity. Length of season and productivity effects showed some species' population growth rates decreasing following longer seasons or higher productivity, opposite our predictions. These results suggest that, for secondary consumers like ducks, the role of growing season characteristics may be mediated or superseded by more complex biotic and abiotic interactions than can be explained with simple annual summaries of growing season characteristics.