Flight Behaviour of Nocturnally Migrating Birds in Coastal Areas: Crossing or Coasting

Abstract

Numerous reports on homogeneous streams of nocturnal migrants over land and water, crossing coastlines without taking any notice of the habitat change underneath, seem to contradict recent infrared observations along the French and Spanish Mediterranean coasts in autumn, suggesting important deviations from the standard SSW or SW flow of migrants associated with the geographical situation and the course of the coastlines. In order to look for potential reactions of nocturnal migrants to the sea/land transition, the flight paths of night-migrating birds were recorded by tracking radar in autumn 1996 at the southern tip of Mallorca and at the E-W leading coast near Malaga. Within the tracking time of 40 s per individual there was no short-term adjustment of direction when crossing the coastline, neither were there differences in the average vertical speeds of birds approaching and leaving the coast. The mean altitudes of tracked birds, however, were about 9% higher above land than over water at both sites. Between-site comparison revealed higher average flight speeds of birds above the island than when leaving the mainland. At both sites important variation in flight behaviour was observed in the course of the night: near Malaga a continuous shift towards the E-W leading coast, simultaneously over water and land, suggests diminishing motivation to continue flights across the sea at times when decreasing fat reserves and/or an endogenous rhythm dissuade from crossing an obstacle of unknown dimensions. Increasing proportions of reverse migration in the course of the night at both sites, with birds flying at low levels and low speeds, are additional indications of motivational conflicts between continuing migration and landing. Optimal migratory behaviour therefore does not necessarily imply that birds should follow the shortest route, but appears to be an adjustable compromise between risk avoidance and straight flight depending on endogenous and environmental conditions.