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| The onset of migration in birds is assumed to
be primarily under endogenous control in longdistance
migrants. Recently, climate changes
appear to have been driving a rapid change in
breeding area arrival. However, little is known
about the climatic factors affecting migratory
birds during the migration cycle, or whether
recently reported phenological changes are
caused by plastic behavioural responses or
evolutionary change. Here, we investigate how
environmental conditions in the wintering areas
as well as en route towards breeding areas affect
timing of migration. Using data from 1984 to
2004 covering the entire migration period every
year from observatories located in the Middle
East and northern Europe, we show that passage
of the Sahara Desert is delayed and correlated
with improved conditions in the wintering areas.
By contrast, migrants travel more rapidly
through Europe, and adjust their breeding area
arrival time in response to improved environmental
conditions en route. Previous studies
have reported opposing results from a different
migration route through the Mediterranean
region (Italy). We argue that the simplest explanation
for different phenological patterns at
different latitudes and between migratory routes
appears to be phenotypic responses to spatial
variability in conditions en route. | |
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