Trailing the heat: Eurasian teal Anas crecca schedule their spring migration basing on the increase in soil temperatures along the route

Migratory behaviour allows individuals to inhabit areas with optimal environmental conditions throughout the year. To reduce energy expenditure and the risk of mortality while migrating, birds may schedule their departures basing on environmental cues that provide seasonal and/or local information. In this study, we aimed to identify the possible effect of environmental factors on the spring migration of 30 Eurasian teal Anas crecca tracked between 2014 and 2018 from Italian wintering areas. We used Cox proportional hazard and generalized estimating equation models to evaluate the environmental cues that affect teal's decision to start migratory movements from the wintering grounds and continue migration from stopover sites. Apart from the anticipated effect of photoperiod, the onset of spring migration was not substantially influenced by environmental variables, whereas the speed of migration seemed to be influenced by both seasonal (increased ground temperature, an indicator of spring advancement) and local (low cloud cover and northward blowing winds, which support migratory flight) environmental cues. The slow migration observed in teal may favour a strategy in which migratory timing is modulated mainly by the conditions encountered during the journey rather than at the start of the migration. This suggested low impact of local environmental variables on the onset of spring migration could have important consequences both for the management of this species for hunting purposes and for the way the species might respond to the ongoing climatic change.