Many bird species call during migration, but call rates not necessarily reflect migration intensity. They rather seem to increase under deteriorating flight conditions. Often, nocturnal mass collisions at illuminated structures coincide with such conditions and are accompanied with high call rates of migrants. Thus, call rates could act as an indicator for situations with high collision risk for birds namely at offshore sites with hardly any alternatives for landing. In the face of increasing numbers of offshore wind farms knowledge about the environmental conditions in which maximum call rates occur, is needed for mitigation measures.

In this first long-term study at an offshore site in the southern North Sea we investigated the effect of weather on the frequency of flight calls of three thrush-species at an illuminated platform. Flight calls were registered automatically during three autumn migration seasons. Besides generally higher call rates from 5 to 2 h before until 6 h after midnight, call rates increased with tailwinds, a change of the tailwind component during the first part of the night, offshore crosswinds and very high humidity.

A monitoring programme is suggested that could help to reduce mass mortalities at illuminated structures.