Species ranges are set by limitations in factors including climate
tolerances, habitat use, and dispersal abilities. Understanding the factors
governing species range dynamics remains a challenge that is ever more important
in our rapidly changing world. Species ranges can shift if environmental changes
affect available habitat, or if the niche or habitat connectivity of a species
changes. We tested how changes in habitat availability, niche, or habitat
connectivity could contribute to divergent range dynamics in a sister-species
pair. The great-tailed grackle (Quiscalus mexicanus) has
expanded its range northward from Texas to Nebraska in the past 40 years, while
its closest relative, the boattailed grackle (Quiscalus major),
has remained tied to the coasts of the Atlantic Ocean and the Gulf of Mexico as
well as the interior of Florida. We created species distribution and
connectivity models trained on citizen science data from 1970–1979 and
2010–2019 to determine how the availability of habitat, the types of
habitat occupied, and range-wide connectivity have changed for both species. We
found that the two species occupy distinct habitats and that the great-tailed
grackle has shifted to occupy a larger breadth of urban, arid environments
farther from natural water sources. Meanwhile, the boattailed grackle has
remained limited to warm, wet, coastal environments. We found no evidence that
changes in habitat connectivity affected the ranges of either species. Overall,
our results suggest that the great-tailed grackle has shifted its realized niche
as part of its rapid range expansion, while the range dynamics of the
boat-tailed grackle may be shaped more by climate change. The expansion in
habitats occupied by the great-tailed grackle is consistent with observations
that species with high behavioral flexibility can rapidly expand their
geographic range by using human-altered habitat. This investigation identifies
how opposite responses to anthropogenic change could drive divergent range
dynamics, elucidating the factors that have and will continue to shape species
ranges.