Stormy Seas for Some North American Songbirds: Are Declines Related to Severe Storms During Migration?

‘‘Many large flocks of birds flew over, coming from the north and flying to the SW. They were more varied in kind than any we had seen before and they were land birds . . . . I know that most of the islands discovered by the Portuguese have been found because of birds.’’—Christopher Columbus, east of Bermuda, 7 October 1492 Several hypotheses have been presented to explain population declines in long-distance migrant songbirds in eastern and central North America (Rappole 1995). Although declines probably result from the interplay of several factors (Blake et al. 1992), habitat destruction on the wintering grounds in the Neotropics and on the breeding grounds in North America is believed to be a major cause of the declines (e.g. Terborgh 1989, Askins et al.1990, Rappole 1995, Sauer et al. 1996). However, the evidence for a causal link between large-scale population declines and habitat destruction is not strong (Petit et al. 1992). Many species of songbirds that breed in eastern North America migrate over the western Atlantic Ocean or the Gulf of Mexico to winter in the Caribbean, Central America, or South America (Williams et al. 1977, Williams 1985, Moore et al. 1995, Nisbet et al. 1995). During these transoceanic flights, migrants tend to avoid periods of unfavorable weather, and large-scale movements often coincide with favorable wind conditions (Richardson 1976, Williams et al. 1977, Stoddard et al. 1983, Alerstam 1990). In contrast, songbirds that breed in western North America migrate over land through Mexico and Central America (Moore et al. 1995), where they can make landfall if wind conditions deteriorate. Many birds succumb during unfavorable weather while attempting to cross the Atlantic Ocean (Williams et al. 1977, Alerstam 1990:337–338) and the Gulf of Mexico (Lowery 1946). Consequently, the abundance of songbirds from eastern North America might be affected more strongly by the frequency of unfavorable weather events during migration than would the abundance of songbirds from western North America, because the latter group does not have to migrate over the ocean. The ‘‘storm hypothesis’’ posits that the abundances of Neotropical species in eastern North America are negatively related to the severity of storms during autumn passage over the ocean. Predictions from the storm hypothesis are that: (1) species that make long flights over the ocean should exhibit greater de-