a1 Conservation And Research Center, National Zoological Park, Smithsonian Institution, Front Royal, Va 22630, U.S.A.
a2 World Wildlife Fund U.S., 1250 24Th Street Nw, Washington, D.C. 20037, U.S.A. Current Address: Global Environment Facility, The World Bank, 1818 H Street Nw, Washington, D.C. 20433, U.S.A.
Over half of the 332 migratory bird species that breed in North America and winter in the tropics are affected by the obstacle to migratory flight presented by the Gulf of Mexico. Landbird migration in the vicinity of the Gulf is considered from an historical perspective, and in light of netting and observational data from the western Gulf coast. A trans-Gulf crossing from the northern Gulf coast to, or over, Yucatan is the most commonly followed fall route for eastern Nearctic migrants that winter in Central America. The spring route for these species is different, involving a more westerly trans–Gulf course for some individuals, and a circum–Gulf route for others. Prevailing wind direction and the probability of meeting turbulence over the Gulf are suggested as the main selective factors affecting route form for Gulf–area migrants.