That, in turn, reduces the available seeds and insects to feed birds, which must work hard to store up enough fat to migrate back to nesting grounds in North America. The thinner, weaker birds that do survive the migration are less fertile, and less likely to reproduce successfully. They found that survival and reproduction rates for these birds were lower than average during El Ni o years and higher during La Ni a years when Pacific Ocean waters are cooler than average. Neotropical migrants such as the black-throated blue warbler breed in North America and spend the winters in Latin America and the Caribbean. In recent years scientists have become alarmed by population declines in many neotropical migrant species.
Most birds breeding in North American forests during the summer are neotropical migrants.
Part of the difficulty in determining the causes of songbird declines has been the inability of researchers to determine where particular birds went during migrations. Sillett, Holmes and Sherry were able to determine that their group of black-throated blue warblers always nests in New Hampshire and winters in Jamaica. The researchers found evidence that the climate changes associated with El Ni o years diminished the birds' food supply, causing low reproductive success in their New Hampshire breeding grounds and low survival rates among adults wintering in Jamaica. In contrast, reproductive success and adult survival were much higher during La Ni a years. Although the El Ni o cycle is a natural phenomenon, many climatologists believe that global warming is escalating the cycle's frequency and severity.