Tracking ice phenology by migratory waterbirds: settling phenology and breeding success of species with divergent population trends

Dependence on climate-driven environmental cues in the initiation of life cycle stages is a critical attribute when assessing vulnerability of species to climate change impacts. This study focused on spring ice phenology as a cue to the settling of migratory waterbirds, asking whether there is an asynchrony between ice phenology and setting phenology that could affect breeding success of six species with divergent population trends. In the 37 study lakes in southeastern Finland, the ice-out date not only varied considerably between years, but became progressively earlier during the study period, 1991–2018. Settling phenology of all species tracked inter-annual variation in ice phenology. However, the degree of asynchrony between ice phenology and settling phenology varied between species, allowing discrimination between early and late settlers. Considerable inter-annual variation also occurred within species, but in only one species did the degree of asynchrony correlate with the ice-ou...