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Cuckoos are unable to change their migration timing in response to climate change

A new study led by the British Trust for Ornithology (BTO) has found that, although spring is arriving faster each year, cuckoos appear unable to change the timings of their annual migration in response.

Before they can cross the Sahara Desert to travel to their European breeding grounds, these birds must wait for the arrival of the west African spring rains. 

However, since the timing of these rains has remained constant, they may arrive in the UK and other European countries out of sync with the peak availability of the invertebrates they prey upon and the breeding ecology of the host species they parasitize.

Most bird species that breed in Europe after spending their winters in sub-Saharan Africa have changed their arrival time in the UK in response to climate change.

Unfortunately, data from the long-running BTO Cuckoo Tracking Project revealed that cuckoos are unable to adapt their behavior in order to avoid the negative impacts a warming planet has on their survival and reproduction rates. 

What the researchers learned 

Before migrating to the UK, cuckoos leave their wintering grounds in the rainforests of central Africa in late February and spend about a month fattening up in west Africa before they embark on their arduous crossing of the Sahara Desert.

According to data from 87 cuckoos tagged since 2011 by BTO experts, the birds must wait here for the explosion of invertebrate numbers occurring each year with the arrival of spring rains. 

However, since spring is currently arriving earlier in Europe, there is a significant mis-match between the cuckoos’ arrival, the peak availability of the caterpillars they feed upon, and the breeding cycles of the host species they usually parasitize. 

Although previous studies have shown that migratory species which are unable to change the timing of their arrival on their breeding grounds face more significant declines, the reasons for this have largely remained unknown, with reduced breeding success appearing to play a surprisingly minor role.

The current study suggests that cuckoos face a greater mortality risk as they rush to return to their breeding grounds in time, leading to worrisome declines in their numbers.

Insights will inform conservation efforts

These findings highlight the need to provide better quality habitats at strategic points along cuckoo migration routes to help them complete their journeys in a more efficient and less energetically costly way, and thus provide them better tools to adapt to a rapidly changing climate.

 “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 follow their return from central Africa. Many other species are thought to be able to bring forward their arrival by adjusting their internal clocks to leave their wintering grounds sooner – but this doesn’t appear to be an option for the UK cuckoo population,” said senior author Chris Hewson, a lead scientist at the BTO Cuckoo Tracking Project.

“Understanding why these are not arriving back earlier – and the possible costs that individual cuckoos pay for trying to do so – will help us to best direct efforts of flyway restoration that may allow them to make their migrations in a more timely and successful fashion.”

The study is published in the journal Proceedings of the Royal Society B.

More about cuckoo birds

The term “cuckoo” actually refers to a diverse group of birds within the family Cuculidae, encompassing approximately 130 species. These birds are native to many parts of the world, including Europe, Asia, Africa, Australia, and the Americas.

Cuckoos are best known for their unique breeding behavior, as some species are what’s called “brood parasites.” This means that instead of building their own nests and raising their own young, these cuckoos lay their eggs in the nests of other bird species.

When the cuckoo chick hatches, it often kicks out the host’s own eggs or young chicks to eliminate competition for food. The host parents continue to raise the cuckoo chick, unaware that it isn’t their own offspring.

In terms of their physical characteristics, cuckoos are typically medium-sized birds with slender bodies. They have long tails and strong, zygodactyl feet (two toes facing forward and two backward). The bill is usually long, thin, and slightly curved.

Cuckoo species can vary widely in their coloration, ranging from brown, gray, and white to more striking patterns in some species. Not all cuckoos are brood parasites, and those that aren’t typically build nests and raise their own young, just like most other birds.

The cuckoo is also famous for its distinctive call, which has been immortalized in clocks (cuckoo clocks) and in music and literature. The European Cuckoo, in particular, has a call that’s widely recognized: a simple, two-note “coo-coo.”


By Andrei Ionescu, Staff Writer

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