Southward Breeding Expansion of Tree Swallows in Alabama

Since first confirmation of nesting in 1987, there has been a steady southward expansion of breeding Tachycineta bicolor (Tree Swallow) in Alabama. Using 220 breeding season occurrence records, we performed a breakpoint analysis to quantify the breeding expansion rate of Tree Swallows in the state. Between 1988 and 2013, our models indicated Tree Swallows expanded similar to 130 km south at a mean rate of 5.2 km/year. Possible drivers of this southward expansion include increased artificial nest box availability, aerial insect declines in more northern historic ranges, changing climate, and altered land-management practices. Our findings indicate Tree Swallows are following a similar southward breeding expansion pattern to that exhibited by Hirundo rustica (Barn Swallow) and Petrochelidon pyrrhonota (Cliff Swallow) in recent decades.