Radar observations of bird migration over the Western North Atlantic Ocean

Abstract

1. During two fall migration seasons, a large number of flying targets were tracked by a radar mounted on an oceanographic research vessel. Obervations were made in September and October in a region extending from the New England coast to the area south and east of Bermuda (Figs. 1 and 3). Accurate local wind measurements were made at short intervals during the observations. 2. Evidence was obtained that many of the targets were birds engaging in migration between coastal North America and the neotropics. Such birds often appeared in discrete ‘waves’ associated with cold fronts passing the North American coast. Distant from land, long periods occurred between ‘waves’ when no targets were observed. 3. Other targets, which were probably also migrating birds, flew in directions which were difficult to explain, or flew at speeds relative to the air which were lower than seem consistent with the energetic requirements of long-range flight. Extrapolations of the possible flight paths of such birds (Figs. 8–10) suggested that birds, even some of those flying at the higher speeds, may take several days in attempting to cross the Western North Atlantic. 4. Birds appeared to fly at higher altitudes during the day than at night, perhaps to allow temperature regulation without evaporative water loss. 5. The results are discussed in relation to previous laboratory measurements of caloric expenditures and flight speeds of birds in wind tunnels, possible use of structure in the atmosphere by small migrants, and the selection pressures which might play a role in the phenomenon of long migrations over water.