Secrets of the night : roost sites and sleep disturbance factors during the autumn migration of first-year white stork Ciconia ciconia

The migration phase of birds is divided into two main states: stopovers and actual migratory flights. For soaring birds, such as storks, nighttime is especially important to conserve energy and to start the next day in favourable weather conditions. Although there is a large number of recent studies on white stork Ciconia ciconia, for example using advanced technologies such as GPS technology, the nocturnal behaviour of the species is still an enigma. Thus, we GSM–GPS-tagged 90 immature storks and checked their nocturnal behaviour, especially roost disturbance, during their first autumn migration from breeding grounds in southern Poland to wintering places in Africa. Storks roosted on three types of site: on buildings, on the ground or in trees. Birds that roosted on the ground had a higher probability of nighttime disturbance than those that used trees or buildings. The probability of disturbance at night was also related to longitude and latitude; the most easterly birds and those at the start of the migration route were disturbed more often during the night. Furthermore, and interestingly, environmental conditions at roosts were also significantly related to the probability of disturbance; birds roosting at tree sites with higher NDVI (greener) and with higher levels of artificial light both had a lower probability of disturbance. A possible explanation of this could be related to lower potential predatory pressure at night. We found that after long flights birds were disturbed more often at night, and that disturbed birds migrated further the next day. For a better understanding of the nocturnal behaviour of storks, as well as of other migratory birds, the use of modern technological tools with greater precision is recommended.