Flight behaviour of sharp-shinned hawks during migration. I : Over land

Abstract

We tested two hypotheses that have been proposed to explain why large numbers of sharpshinned hawks ( Accipiter striatus) are counted during fall migration at Cape May Point, New Jersey. The most popular hypothesis, which suggests that hawks are drifted to the coast by west to northwest winds, was rejected in favour of the alternative hypothesis, which suggests that the large numbers of hawks seen on west winds resulted from a sampling bias. Using modified marine radar, we found that sharp-shinned hawks flew significantly lower at Cape May on days with west winds than on days with other winds, making them easier to count. The altitude of flight on days with other winds was regularly greater than 400 m, and hawks were difficult to detect without the radar. Migration traffic rates at Cape May were consistently greater than the broad-front migration rates computed from counts taken 36 km north and inland from Cape May. Flight directions measured at the inland site, away from topographic leading lines, showed that sharp-shinned hawks compensate for different wind directions by adjusting their headings, and the direction realized on all winds brings them to the coast. Our results suggest that counts of migrating hawks at some topographic features are subject to systematic biases and the conclusions derived from these counts may be erroneous.