In autumn 1998, nocturnal bird migration at Falsterbo was recorded over a p
eriod of three months by means of a passive infrared device. At the same pl
ace a standardised trapping scheme is in operation. This study reveals for
the first time a positive correlation between the migratory intensity of bi
rds aloft and the numbers of trapped birds. By relating the migratory patte
rns of single species with the nocturnal. migratory intensities, we show th
at the species composition aloft can at least partly be deduced from the nu
mber of grounded birds. The numbers of trapped pre-Saharan migrants were re
lated more strongly to the migratory intensity of the preceding night than
were the numbers of trans-Saharan migrants. Assuming that the number of tra
pped birds varies according to the weather to the same extent as the migrat
ory intensity of birds aloft, we conclude that in shorter range migrants th
e decision to engage in a migratory flight is influenced more strongly by w
eather conditions, and that the migratory activity of trans-Saharan migrant
s is possibly more intensely controlled by their endogenous. migratory urge
.