Phenological shifts are well-documented in the ecological literature.
However, their significance for changes in demography and abundance is
less clear. We used 27 years of citizen science monitoring to quantify
trends in phenology and relative abundance across 89 butterfly species.
We calculated shifts in phenology using quantile regression and shifts
in relative abundance using list length analysis and counts from club
trips. Elongated activity periods within a year were the strongest
predictor of increases in relative abundance. These changes may be
driven in part by changes in voltinism, as this association was stronger
in multivoltine species. Some species appear to be adding a late-season
generation while other species appear to be adding a spring generation,
revealing a possible shift from vagrant to resident. Our results
emphasize the importance of evaluating phenological changes throughout
species' activity periods and understanding the consequences for such
climate-related changes on viability or population dynamics.