Diurnal migration patterns in willow warblers differ between the western and eastern flyways.

Area of Science:

• Ornithology
• Ecology
• Animal Behavior

Background:

• Migratory divides in birds are thought to be maintained by selection against intermediate migratory directions.
• Empirical evidence supporting this hypothesis, particularly in passerines, remains limited.

Purpose of the Study:

• To investigate migratory behavior in willow warblers at a migratory divide in central Sweden.
• To test the hypothesis that birds using intermediate migratory routes are forced to fly more during the day when crossing ecological barriers.

Main Methods:

• Analysis of geolocator tracking data from willow warblers breeding across Sweden.
• Comparison of migratory timing (day vs. night flight) across different migratory routes (SW, SE, and intermediate).

Main Results:

• Contrary to the hypothesis, no increased daytime migration was observed for birds on intermediate routes.
• Birds migrating along the western flyway were most likely to undertake full-day flights.
• The probability of full-day migration decreased linearly from west to east across the migratory divide.

Conclusions:

• The study challenges the long-standing view that selection against intermediate migratory routes drives narrow migratory divides.
• Western flyway migrants' propensity for daytime flight may be linked to favorable tailwinds or challenging conditions in the western Sahara.
• Further research is needed to fully understand the drivers of migratory behavior and route selection in willow warblers.