Open ocean bird migration

A large body of indirect evidence suggests that migrant birds may constitute a significant proportion of the coherent airborne radar echoes detected at sea. Shipboard and island-based radars frequently detect large numbers of small cross-section targets moving at angles to the wind. Calculated airspeeds of these echoes are between 25 and 100 km/h (7-28 ms^-1). Radar ornithologists have concluded that these echoes are due to migrant birds as the peak density occurs during fall and spring migration seasons. Land birds which make energetically advantageous crossings between continents are the most numerous migrants detected on radar. The occurrence, speed, and direction of the migrations are dependent not only upon location but also on the season of the year and weather conditions prevailing at the continental point of departure. Avian radar echoes over the open oceans can usually be identified as waterfowl, shorebirds, passerines or seabirds. The first three groups, which are all land birds, are most dense near continental margins, but significant migrations have been detected by radar more than 3000 km from land. Seabirds have not been extensively studied with radar but are known to migrate at least in small numbers over all the oceans of the world