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Migration can take place primarily during the day, or at night, or during
both depending on the species. Why the timing of migration varies among
species has been the object of much research but the underlying ecological
processes are still unclear. Proximally, migration timing may be
influenced by the timing of other activities or may be more prevalent in
species that migrate over long distances. Adaptive scenarios emphasize the
reduction in flight costs at night especially in smaller species and the
advantages of travelling in groups during the day to locate staging sites
more efficiently. I used phylogenetic independent contrasts to examine
these hypotheses in all North American nesting birds. I uncovered 24
evolutionary transitions in migration timing, most of which involved a
switch from nocturnal to diurnal migration. Few of these transitions
involved a concomitant change in the timing of foraging habits or
migration distance. However, species in diurnal clades were larger,
travelled in larger flocks and were generally more sociable than their
nocturnal counterparts. The results support the hypotheses that a
reduction in flight costs and the ability to pool information from
companions are associated with migration timing in North American bird
species.
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