Have the Birds Flown?

Migration is very energetically demanding, especially for small birds. Such species only do this when the costs versus benefits analysis results in gains (in terms of avoiding fatal cold snaps and/or being able to forage for longer). It had been assumed that birds undertaking migrations timed their movements purely in relation to changing daylength. A recent analysis by workers at Durham University of 50 years of migration/climate/vegetation data suggests, however, that this is not the whole story (https://www.theguardian.com/environment/2021/oct/25/britains-migratory-birds-may-stop-flying-south-for-winter). Some small birds that breed in the UK (notably the Willow warbler; the Garden warbler and the Nightingale) seem to be spending less time in overwintering sites such as Africa and Southern Europe (in some cases over 50 days less). Recently, birds have been leaving later to travel South and earlier to migrate back to the UK. If the trends continue, some species might even dispense with the annual migration altogether. These small birds appear to be able to adjust their migrations in response to other factors such as prevailing temperatures and food availability. Yet another clear illustration of the impact of global heating?