Corn or Climate? Climate Change Facilitated, but Did Not Initiate, the Northward Range Expansion of the Green Jay in Texas.

Abstract

Anthropogenic changes are driving global shifts in species distributions. Two highly divergent bird species, the green jay (Cyanocorax yncas) and the blue jay (Cyanocitta cristata), recently hybridized in Texas following range expansions. While the southwestern spread of the blue jay has been attributed to suburban bird-feeding, the cause of the green jay's > 2° northward expansion over the last 70 years is less clear. Climate change has been proposed as a driver, but concurrent changes in habitat and resource availability-including widespread supplemental feeding of deer-complicate interpretation. We combine ornithological records, eBird observations, climate data, and economic data to investigate the progression of the green jay's range expansion in relation to climate and supplemental feeding. The first records of resident green jays north of southernmost Texas occurred in the 1960s. Sustained northward expansion was detectable from 1980 to 2021 and coincided with reduced severe freeze frequency from 1982 to 1996. Severe winters occurred alongside a reduced probability of green jay detections in newly occupied locations. However, range expansion increased during the period with the coldest average winter in the 95-year climate record and coincided with growing supplemental feeding. We suggest that winter food supply on widespread hunting ranches and habitat loss drove initial northward colonization by green jays. Subsequent warming may have then relaxed climatic constraints once this novel resource became widespread. This potential interaction between novel resources and climate change highlights that different anthropogenic mechanisms may work together to alter species distributions.