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Cuckoos are no match for local reed warblers
By Ella Davies
Earth News reporter

Reed warbler nest (Image: Bard Stokke)
Spot the difference: Cuckoos cannot mimic local variations in egg appearance

There is a limit to how convincingly the common cuckoo can mimic other birds, according to scientists.

Previous research found that cuckoos adapt the appearance of their eggs to match those of birds whose nests they invade.

A new study shows that their imitation is not sophisticated enough to mirror local variations in reed warbler eggs.

A cuckoo's ability to hide its eggs in an existing clutch is integral to its parasitic method of parental care.

Common cuckoos (Cuculus canorus) are famed for laying their eggs in host species' nests, leaving unwitting "foster" birds to raise their chicks.

They have evolved specialisation but they have not evolved local adaptation
Jesus Aviles

Known as "brood parasites" cuckoos are able to specialise their eggs' appearance in order to disguise them in the nests of other birds.

However, in their paper published in the Journal of Evolutionary Biology, researchers reveal the limit of cuckoo mimicry.

"They have evolved specialisation but they have not evolved local adaptation," says the paper's co-author Jesus Aviles.

"They can mimic well the eggs of different species, which are very different, but they have more problems evolving good local mimics within a single species," he explains.

In their study, the researchers observed differences in the size and colouration of reed warbler (Acrocephalus scirapaceus) eggs in different locations.

Cuckoo eggs in these reed warbler nests are similar in appearance but do not perfectly match the variations.

This evidence suggests that cuckoos are not locally adapted to their reed warbler hosts.

Reed warbler feeding cuckoo chick (c) Andy Sands / NPL
Reed warblers cannot distinguish cuckoo chicks from their own

A raft of previous studies have identified an ongoing co-evolutionary "arms race" between cuckoos and reed warblers.

Reed warblers evolved the ability to identify cuckoo eggs by sight which led to cuckoos adapting the appearance of their eggs.

Reed warblers were also found to eject cuckoo eggs from their nests if they were more or less advanced in development than the existing clutch.

Cuckoos were observed watching reed warbler nests in order to take advantage of feeding runs by egg-developing female reed warblers.

The parasitic species also evolved rapid laying to capitalise on these opportunities.

BLIND ASSASSIN
A cuckoo chick ejects its nestmates

Male reed warblers were studied standing guard at nests while females laid and cared for eggs. The males aggressively defended their nests, mobbing potential cuckoo intruders.

Earlier this year, cuckoos were found to incubate their eggs internally for 24 hours before laying them, effectively giving their offspring a head start on host nest-mates.

Nest invasion

The presence of a cuckoo chick in its nest has devastating consequences for the reed warbler.

Once hatched, the cuckoo chick ejects any other young from the nest.

Reed warbler parents are not able to distinguish cuckoo chicks from their own and will work hard to feed the comparatively giant chicks.

So it would be advantageous for the reed warblers to evolve the ability to exploit any limitations to the cuckoos' ability to trick them.

SOURCES

However, Mr Aviles asserts that local variations in reed warbler eggs cannot be considered a defence against cuckoo parasitism.

"These variants have more likely evolved due to population differences in environmental factors affecting egg colouration," he tells the BBC.

In an earlier study, Mr Aviles identified a correlation between reed warbler egg colour and environmental factors including rainfall and temperature.

His work suggests that environmental influences play a key role in the ongoing evolutionary competition between cuckoos and reed warblers.



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