B. Bruderer et F. Liechti, Flight behaviour of nocturnally migrating birds in coastal areas - crossing or coasting, J AVIAN BIO, 29(4), 1998, pp. 499-507
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 Fren
ch and Spanish Mediterranean coasts in autumn, suggesting important deviati
ons from the standard SSW or SW flow of migrants associated with the geogra
phical situation and the course of the coastlines.
In order to look for potential reactions of nocturnal migrants to the sea/l
and 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 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 ap
proaching and leaving the coast. The mean altitudes of tracked birds, howev
er, were about 9%, higher above land than over water at both sites. Between
-site comparison revealed higher average Right speeds of birds above the is
land than when leaving the mainland. At both sites important variation in f
light behaviour was observed in the course of the night: near Malaga a cont
inuous shift towards the E-W leading coast, simultaneously over water and l
and, suggests diminishing motivation to continue flights across the sea at
times when decreasing fat reserves and/or an endogenous rhythm dissuade fro
m crossing an obstacle of unknown dimensions. Increasing proportions of rev
erse migration in the course of the night at both sites, with birds flying
at low levels and low speeds, are additional indications of motivational co
nflicts between continuing migration and landing. Optimal migratory behavio
ur therefore does not necessarily imply that birds should follow the shorte
st route, but appears to be an adjustable compromise between risk avoidance
and straight flight depending on endogenous and environmental conditions.