Breeding latitude and timing of spring migration in songbirds crossing the Gulf of Mexico
Each spring, millions of songbirds migrate across the Gulf of Mexico on their way to breeding sites in North America. Data from radar and migration monitoring stations have revealed broad patterns in the spatial and temporal course of trans-Gulf migration. Unfortunately, we have limited information on where these birds have previously spent the winter and where they are migrating to breed. Here we measure stable-hydrogen isotopes in feathers (delta D(f)) to infer the breeding latitude of five species of songbirds - hooded warblers Wilsonia citrina, American redstarts Setophaga ruticilla, black-and-white warblers Mniotilta varia, ovenbirds Seiurus aurocapilla, and northern waterthrushes S. noveboracensis - that were captured at a stopover site along the coast of southwestern Louisiana in spring 2004. Values of delta D(f) across all species ranged from -163 to -35 parts per thousand (n=212), and within most species the range was consistent with the latitudinal extent of known breeding sites in central and eastern North America. Individuals that arrived first along the northern Gulf coast had delta D(f) values indicative of southerly breeding sites in hooded warblers, American redstarts, black-and-white warblers, and ovenbirds, but no relationship was found between passage timing and delta D(f) for northern waterthrushes. Our findings suggest that spring passage is often timed to coincide with the emergence of suitable conditions on breeding areas, with southern breeding birds migrating first.