Latitude shifts in eastern North American waterbird distribution tracks winter temperature gradients

Background/Question/Methods

Species are responding to climate by shifting their geographic ranges. Poleward shifts are common, however, some species are not shifting their range in this way. If species fail to track their climatic niche, this could lead to population-level consequences. Quantifying patterns of range shifts are important for predicting climate-driven extinction risk. There is currently a lack of direct study on winter range shifts in North American waterbirds, despite the effects of climate change disproportionately affecting them (e.g. through sea-level rise). In this study, we test if there is an association between winter temperatures and winter range shifts in coastal and inland waterbirds. We use The National Audubon Christmas Bird Count, a geographically widespread community science dataset, to calculate a weighted center of abundance for 101 species in eastern North America from 1990–2018. For each year we constructed the 0ºC isotherm from averaged December and January temperatures from the WorldClim database. We used linear regression to determine if species significantly shifted their annual weighted latitudes north or southward. We constructed generalized linear mixed models to test if distance shifted was related to the position of the 0ºC isotherm, minimum winter temperature, and preferred habitat with species as a random effect.

Results/Conclusions

Of the 101 species, 46 species (45.5%) shifted in their respective weighted latitude. We found that 34 (33.6%) species shifted their weighted latitude significantly northward and 12 (11.8%) species significantly southward. Fifty-five (54.4%) species that did not significantly shift in latitude. On average, species that have shifted northward in latitude did so at a rate of ~4.30 km per year, while species shifting their weighted latitude south at a rate of ~5.70 km per year. Our general linear mixed model revealed a significant negative association between weighted latitude and minimum winter temperature, but no association with position relative to the 0ºC isotherm. For species that were considered to be mainly inland-distributed, we found a relationship between position of the 0ºC Isotherm and their shifts. This is the first study to explicitly examine geographic range shifts in North American waterbirds relative to winter temperatures and habitat type. This information will be valuable to resource managers preparing for changes in distribution, harvest regulations, and location of protected areas.