New research shows how camouflage protects nesting birds from being eaten
LINKED PAPER
Camouflage predicts survival in ground-nesting birds. Troscianko, J., Wilson-Aggarwal, J., Stevens, M. & Spottiswoode, C.N. 2016. Scientific Reports. DOI: 10.1038/srep19966. View
Camouflage is everywhere in nature, used by prey to avoid being seen by predators and by predators to sneak up on their victims. Concealing coloration is, overall, probably the most widespread means of avoiding predation. However, demonstrating direct evidence for the effectiveness of camouflage in the wild has been difficult. Our recent study shows that camouflage is a vital route to survival in a variety of avian species.
Birds exhibit some of the most impressive examples of camouflage found anywhere in nature (Fig. 1). One of its main uses is to protect the nests of ground-nesting species like plovers and nightjars from being detected and attacked by predators. In nightjars, the adults can be spectacularly well matched to their environment, hiding their eggs when sitting on the nest. In birds like plovers and coursers, the eggs themselves often resemble the ground so closely that they are notoriously hard to see.
Figure 1 Top: a Mozambique nightjar (Caprimulgus fossii) with its well camouflaged plumage. Bottom: eggs of the bronze-winged courser (Rhinoptilus chalcopterus), which appear well matched to the recently burnt earth. Both images © members of Project Nightjar. Click on image for larger version
Camouflage is in many ways an intuitive concept, and partly for that reason most work until recently was rather subjective and observational in nature. Few experiments had been conducted to test if camouflage actually does work and how effective it is, especially in wild free-ranging animals. Those studies that had been conducted were often limited in scope too. For example, research testing whether the coloration of bird eggs reduces the risk of being seen by predators has provided equivocal results, in part because it often involved using painted eggs that may provide odour cues to predators, and also because past work rarely considered the vision of predators. So it may sound surprising, but until recently there was actually little evidence avian camouflage actually worked.
A few years ago we set up a project (‘Project Nightjar’), led by myself and Claire Spottiswoode (at the University of Cambridge), to determine if and how camouflage works in wild ground-nesting bird species from Zambia and South Africa. The bulk of the work has been conducted by Jolyon Troscianko and Jared Wilson-Aggarwal (both at the University of Exeter), and one of our main aims was to determine whether how closely birds and their eggs matched the background, to the eyes of predators, directly reflected how likely they were to be attacked.
To test this, we first set about finding as many nests of nightjars, plovers, and coursers as we could, relying on the considerable help of local farm labourers in Zambia who were adept at finding nests in the bush. Next, we used special cameras that were capable of recording the colours and patterns of the adult nightjars and the eggs of all species throughout the range of light wavelengths that potential predators might see (including ultraviolet light). This allowed us to take images of the birds and their environment without disturbing them too much, which we would later use to analyse how well the birds and their eggs matched the background. Next, we set up camera traps and revisited the nests at set time intervals to see what might be attacking the birds (what the predators are), and which nests survived and which were eaten.
The camera traps not only allowed us to determine the predators that were targeting the nests, but also allowed us to deduce what the main visual systems of the predators would be. We determined that the nests were attacked by three main types of predator. The first were primates like vervet monkeys, which have a colour vision system quite similar to humans, with three types of cone cell sensitive to short- (‘blue’), medium- (‘green’), and longwave (‘red’) light (a trichromatic system). Then there were animals like mongoose, which are dichromatic and see just short and medium/longwave colours (like seeing the world in blue and yellow). Finally, birds themselves were also predators, and see colours using four types of cone cell: red, green, blue, and ultraviolet (tetrachromatic).
Figure 2 Camera trap stills of banded mongoose and a vervet monkey eating the eggs of nesting birds in the study. Both images © members of Project Nightjar.
Back in the UK we went through all the thousands of photographs and used special image analysis techniques to work out how camouflaged the birds were to each of the main predator visual systems for colour, pattern, and brightness. We then related this camouflage data to whether each nest survived or not in the field, and for how long.
The results were, overall, exactly as we expected, with camouflage providing a strong survival advantage against all predators. But there were interesting differences between the bird groups too. Plovers and coursers flee the nest early when a threat approaches, exposing their eggs to be seen for long periods. Correspondingly, the camouflage of their eggs was what mattered in whether or not the clutch survived to hatch. Nightjars, on the other hand, generally sit tight, even when a threat comes very close, and in line with this it was the camouflage of the adult birds not the eggs that influenced whether the nest survived or not.
These results might sound obvious, but they’re actually the first direct evidence that the level of camouflage of animals in the wild, to the vision of their predators, affects the likelihood of survival. While camouflage is intuitive, it has not always been easy to demonstrate its effect, no doubt partly because finding and monitoring camouflaged animals in the field is, by definition, difficult!
Birds are now proving to be a terrific group of animals to understand how camouflage works and its value. Only a few years ago a lab study of Japanese quail showed that mothers choose backgrounds on which to lay that provide the best match to their own individual egg colours (Lovell et al. 2013). Furthermore, other recent work has shown that blue-footed boobies, which lay bright blue eggs, coat their eggs in dirt over a period of days in order to better hide them from predators (Mayani-Parás et al. 2015). So the behaviour of birds has a major influence on how their camouflage works and evolves.
If you want to try out just how hard being a predator can be, and search for hidden nightjar and plover eggs yourself, then try playing these games we made as part of the project – good luck!
References
Troscianko J., Wilson-Aggarwal J., Stevens M. & Spottiswoode C.N. (2016) Camouflage predicts survival in ground-nesting birds. Scientific Reports 6: 19966. View
Lovell P.G., Ruxton G.D., Langridge K.V. & Spencer K.A. (2013) Egg-laying substrate selection for optimal camouflage by quail. Current Biology 23: 260-264. View
Mayani-Parás F., Kilner R.M., Stoddard M.C., Rodríguez C. & Drummond H. (2015) Behaviorally induced camouflage: a new mechanism of avian egg protection. American Naturalist 186: E91-E97. View
Image credit
Featured image: A Fiery-necked Nightjar (Caprimulgus pectoralis) from Zambia, camouflaged against the leaf litter © Martin Stevens
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