Wednesday 25 April 2018

Camouflage proves key to nesting birds' life-and-death decisions

Nightjars are well camouflaged to avoid predators (RSPB)
Nightjars are well camouflaged to avoid predators (RSPB)

Camouflage influences the life-and-death decisions made by nesting birds, according to a new study.

They time their escape from an approaching predator depending on how well hidden they and their eggs are.

The study is believed to be the first to show that the camouflage of an animal or that of its offspring can explain the variation in risk-taking behaviour when approached by a predator.

Researchers from Exeter and Cambridge universities studied the nests of several species of ground-nesting birds in Zambia.

They monitored the nests' progress, recording the escape distance of the adult bird each time they approached and, using camera traps, identified key predators such as banded mongooses, vervet monkeys and grey-headed bush shrikes - and even humans.

The researchers set out to test whether the distances at which birds fled from their nests on the exposed ground was related to the camouflage of their plumage and eggs.

They found that birds which usually flee from predators at long range, such as plovers and coursers, stayed on their nest for longer when the pattern of their eggs was a better match to the background.

They also adjusted their behaviour in the heat of the middle of the day, letting a predator approach a little closer before fleeing.

By contrast, another group of birds, the nightjars, usually sit tight as predators approach so that their eggs are concealed by their camouflaged bodies until the last minute.

Nightjars stayed on their nests longer when the colour and pattern of their own plumage, rather than that of their eggs, was a better match to the background.

The team photographed the adult birds and eggs using specially-calibrated digital cameras.

Study author Jared Wilson-Aggarwal, a PhD researcher, said: "Plovers, coursers and nightjars nest on the bare ground during the Zambian dry season.

"Temperatures can get very high and if approached by a predator the adult bird has to make a hard decision to either, sit tight and continue shading their eggs or to flee the nest and prioritise their own survival.

"Our results suggest that camouflage is able to mitigate not only predation risk but also thermal risks, by permitting adults to shade their eggs for longer when the risk of them overheating is highest."

Martin Stevens, who co-led the project with Dr Claire Spottiswoode, said: "Our study shows how animals monitor their own camouflage and that of their offspring, and use this to guide how they behave.

"It complements a small but growing number of studies showing how important behaviour is in facilitating camouflage in nature."

:: The study, Escape Distance In Ground-Nesting Birds Differs With Individual Level Of Camouflage, is published in the journal The American Naturalist.

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