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.