When you spend your days nesting on the ground and weigh less than 100 grams, self-preservation depends on mastering the art of disguise. The nightjar’s soft plumage and variegated colouring help it blend in with its surrounds, but it is the bird’s own judgement in choosing the most sympathetic background that makes it a camouflage champion.
Each bird chooses where to nest based on its specific patterns and colours, says camouflage researcher Martin Stevens, of the Centre for Ecology and Conservation at the University of Exeter in Cornwall. “Each individual bird looks a little bit different,” he says. “This is not a species-level choice. Individual birds consistently sit in places that enhance their own unique markings, both within a habitat and at a fine scale with regards to specific background sites.”
Stevens is a lead researcher with Project Nightjar, a collaboration between the University of Exeter and the University of Cambridge’s Behavioural Ecology Group to better understand the role of camouflage in the wild. The project team’s latest study, published in Nature Ecology and Evolution, looked at nine species of ground-nesting birds – including nightjars, plovers and coursers – to determine that individual birds make personal choices in choosing nesting spots that improve their camouflage.
Previous research by the group, published in Scientific Reports, has shown the particular importance of camouflage to nightjars both for their own self-preservation and the survival of their offspring. Other species of ground-nesting birds, such as plovers and coursers, will flee their nest when predators approach. Those birds tend to have mottled eggs that more closely match their background. Nightjar eggs, on the other hand, lack their own camouflage, so their concealment from predators depends on the effectiveness of the cover provided by their mothers, who will sit tight in the face of approaching danger.
While it is not yet clear how individuals choose places to suit their appearance, the evidence suggests the birds must have a sense of self-knowledge and of how they relate to their environment. “It could be that somehow they ‘know’ what they look like and act accordingly,” Stevens says. “They may look at themselves, their eggs and the background and judge whether it’s a good place to nest, or learn over time about what kinds of places their eggs escape being eaten.”
Project co-leader Claire Spottiswoode, of the University of Cambridge, says the research is helping to better understand how behaviour and appearance are linked. “We tend to think about camouflage as something that involves gradual evolutionary change in appearance,” she says. “We don’t often think of it as a matter of individual animal behaviour.”
Nightjars, found throughout the world, are divided into three subfamilies: the Caprimulginae, or typical nightjars: the Chordeilinae, or nighthawks, found in the Americas; and Eurostopodidae, or eared nightjars, found in East Asia and Australasia.
The Project Nightjar team, though, does most of its bird watching in Zambia, with the help of locals. “These findings were made possible,” Spottiswood says, “by the amazing field skills of our team of nest-finders from the local community in Zambia, who found hundreds of beautifully camouflaged nests.”