Small-scale migratory behavior of three facultative soaring raptors approaching a water body: a radar study investigating the effect of weather, topography and flock size

Abstract

Water bodies are considered a barrier to the migration of large bird species, mainly because of the absence of thermals that these birds heavily rely on to move large distances with little energy expenditure. In this two-year study, we combined vertical and horizontal radar data with visual observations to compare the autumn migratory behavior of three facultative soaring species: European honey buzzards Pernis apivorus, western marsh harriers Circus aeruginosus and black kites Milvus migrans. Here we used non-parametric tests, linear and generalized linear models to investigate the effect of flock size, age, local weather conditions, time of the day and topography on the small-scale flight behavior of these species, quantified in terms of flight altitude, flight direction and distance from the mountain ridge. European honey buzzards, both adults and juveniles, were detected over the plateau near the mountain chain during suitable weather conditions for soaring flight (especially high temperature) and during high species flow, which facilitated the location of thermals. In contrast, inexperienced juveniles were less concentrated in space, forming smaller flocks and flying at lower altitudes, probably being less facilitated than adult in exploiting the soaring flight. The Western marsh harrier, a raptor largely using the flapping flight even over land, flew lower than adult honey buzzards and nearer to the mountain ridge during strong tailwinds, perhaps being efficient in exploiting their support using the flapping flight even during inter-thermal gliding. Such as western marsh harriers, black kites flew nearer the mountain chain during strong tailwinds, but they probably use soaring flight during such weather conditions to exploit their onward support even when circling in thermals.