Climate change is not to blame for North American migratory birds shrinking in size, a new study claims.
But now the team reveal that birds are changing in size and shape independently of changes in their migration timing. .
'We found that birds are changing in size and shape independently of changes in their migration timing, which was surprising.' .
'Phenological changes' – shifts to the timing of the annual cycles of animals, such as migration times – are generally seen as due to climate change. .
Multiple studies have demonstrated that birds are migrating earlier in the spring as the world warms, leading to 'evolutionary pressure' to migrate faster and arrive at breeding grounds earlier.
This still could be the case – but the new results contradict the suggestions that earlier migration and body size changes are linked.
For this new study, researchers tested for a link between the changes in size (morphological changes) and earlier spring migration – what's known in biology as a 'phenological change'.
Multiple studies have demonstrated that birds are migrating earlier in the spring as the world warms
'Meanwhile, the birds have been changing the timing of their migratory patterns in ways that were imperceptible until the dataset as a whole was examined.'
But the previous study did not test to see whether the changes in body size and wing length were driven by climate-related shifts in migration timing.
'Phenological change' refers to changes to the timing of the annual cycles of animals, such as migration times, while 'morphological' refers to body shape and size.
In other words, earlier migratory behaviours due to climate change are not to blame for the smaller body sizes, as observed in the 2019 study.
Advances in phenology, such as flowering plants blooming earlier in the spring, and changes in morphology, including body size reductions, are among the most commonly described biological responses to global warming temperatures.
Many studies of plant and animal adaptive responses to climate warming have looked at either phenological or morphological changes, but few have been able to examine both at the same time.
'It is often assumed that morphological changes driven by climate and changes in the timing of migration must interact to either facilitate or constrain adaptive responses to climate change,' said Weeks.