Cuckoos are at risk because they can’t adjust their clocks to climate change, new research finds.
The birds belong to several species that migrate from sub-Saharan Africa to European breeding grounds in spring.
With spring arriving earlier and earlier in Europe, most species have adjusted their arrival times to ensure they get the best breeding grounds and prey.
But cuckoos remain in West Africa awaiting the arrival of spring rains, which haven’t shifted their start times as they take the invertebrates they eat with them, researchers from the British Trust for Ornithology (BTO) have found.
The time spent fattening up before the journey is essential to give the birds the energy they need to make the migration to Europe. Cuckoos that make the journey in time for European spring have been found to have higher mortality rates, researchers said.
But arriving late to Europe is also dangerous for cuckoos, as they miss the peak time for caterpillars, their main prey, and are left with the last choice of nesting sites.
Cuckoo numbers are falling in England
The two factors may go some way to explaining why cuckoo numbers have fallen by 71 percent in England since 1995.
The BTO said providing good stopping points at strategic locations along cuckoo migration routes could help more birds survive the journey when they are low on energy reserves.
BTO scientists analyzed data from 87 cuckoos they’ve tagged since 2011 for the study, which was published in the journal Proceedings of the Royal Society B.
Before the study, little was known about where cuckoos went when they left the UK after their breeding season ended in June. Cuckoos are one of the first birds to leave the UK for the winter as they lay their eggs in other birds’ nests and never meet their young.
“It’s fantastic to have this new insight into what determines the spring arrival of our cuckoos, 12 years after the tracking project first allowed us to track their return from Central Africa,” said Dr Chris Hewson, lead scientist of the BTO Cuckoo Tracking Project.
“Many other species are thought to be able to advance their arrival by adjusting their internal clocks to leave their wintering grounds earlier – but this does not appear to be an option for the British cuckoo population.
“Understanding why these aren’t coming back sooner — and the potential costs individual cuckoos pay to try — will help us best direct flight path restoration efforts, helping them make their migrations faster and more successful.”
Other migratory birds such as nightingales and willow warblers have had much more success adapting to the earlier spring season. The change is so extreme, with the birds spending up to two months longer in Europe, experts have suggested migration could be a thing of the past for some species.