Factors affecting the visible southbound migration of raptors approaching a water surface

Abstract

We investigated the visible migration of raptors when approaching a water surface in relation to local wind conditions, time of the day and topography. Observations were made during autumn migration in 2011 and 2012 at a watchsite located in the southernmost part of the Calabrian Apennines (southern continental Italy). In this area, the Apennines show differential topographical features with a plateau west of the mountain ridge. West–southwest of the plateau lies the Straits of Messina, the narrowest water surface between southern continental Italy and eastern Sicily. The passage of migrants was recorded along the ridge of the mountain chain and over the plateau. As expected for a soaring raptor, European Honey Buzzards were detected mostly during midday and early afternoon, while significant numbers were seen toward sunset probably when flying at lower altitude to roost. In addition, they were detected during strong winds when exploiting deflection updrafts, avoiding wind drift and limiting the negative effect of head winds. Western Marsh Harriers were detected mostly during crosswinds blowing from W–NW, perhaps because of a partial drift effect on birds passing along the Tyrrhenian coast. Black Kites were seen concentrating the passage over the plateau, confirming the hypothesis that these raptors tend to move along the western slope of the Italian peninsula perhaps because of foraging behaviour. Finally, European Kestrels/Lesser Kestrels were seen toward the sunset. This result seems to confirm that these small falcons migrate at considerable altitudes resulting in them being hardly detectable by direct visual observation but becoming more visible toward sunset, when they were probably moving to roost.