European woodcocks have the brightest feathers known to exist

With their mottled brown, beige and black plumage, the woodcock is the embodiment of camouflage. Hunkering down in the leaf litter of a forest floor they all but vanish.

But hidden amongst its drab exterior is something of a surprise: the brightest feathers ever seen. 

A new study reveals that the feathers on the underside of woodcocks' tails are the whitest white ever measured. For a bird that is typically active around dawn and dusk, it might seem counter-intuitive for these animals to have such conspicuous colouration, but scientists suspect that it allows them to make a controlled signal to each other before safely hiding it away.

Dr Alex Bond, the Principal Curator and Curator in Charge of Birds at the Museum and co-author of the study, says, 'If you look at woodcocks from the back, they are very camouflaged, very dull brown and mottled. But when you turn it around and spread its tail feathers it is amazing and white.'

'And when you actually measure these feathers, it is the whitest feather that anyone has ever measured. They have the greatest reflectance.'

After initially noticing the bright feathers in the Eurasian woodcock, the scientists found that all eight closely related species also had them. What's more, the structure of these fantastically white tail feathers is highly unusual in that they are arranged a little like a venetian blind.

The study has been published in Journal of the Royal Society: Interface.

A master of disguise

Woodcocks are a small group of similar looking wading birds. They have slender legs, a plump body and a long bill that is perfect for probing in soft mud and soil for all sorts of invertebrates to eat.

But unlike most waders, woodcocks tend to live more inland, frequenting the margins of fields and forests. In these environments, it pays for the birds to dress in browns and blacks that blend in perfectly with their surroundings.

The Eurasian woodcock is found across much of Europe and Asia, often breeding in the north during summer before taking a winter break in warmer climes.

It is during this summer breeding season when the birds need to become more obvious in the hunt for a mate. Rather than staying hidden in the undergrowth, the birds will vocalise while flying over the woodland flapping their wings in a distinctive pattern in a behaviour known as 'roding'. When sitting on the ground they will also flick their tails up and down at potential mates.   

'They are a really active, crepuscular bird that makes these really fantastic whooshing noises in the forest if you go out around dusk in the summer,' explains Alex. 'They are absolutely gorgeous.'

'Our traditional thought about how those birds communicated and signalled was that it was either done vocally or chemically, so either by making a noise or through smells.'

But it was whilst looking at the tails of woodcocks that Jamie Dunning, a researcher at Imperial College London, started to wonder about the white spots found underneath them. While it was already thought that they might play a role in courtship behaviour, no one had really looked at it in much detail.

'It is one of those things that was noted in the 1960s in a Russian paper I think, and like so much of the non-English literature, just didn't get picked up and nobody really thought about it,' says Alex. 'But also it's not somewhere that you would think to look, on the underside of the tips of the tail.'

The whitest white

The team were able to measure the brightness of the white by using modern electron microscopy, spectrophotometry and optical modelling. 

The results showed that the reflectance of the woodcocks' white feathers were an astonishing 30% higher than any previously measured feathers. Detailed analysis of the feathers showed that it is a mixture of the solid structure of the feathers, the air pockets they contain, and the angle of the structures that all contribute to this brilliance.  

Even when compared to other bird species that we might assume to be sporting in even brighter whites, such a snowy owls, Caspian terns or white-crowned manikins, the woodcocks still came out on top.

What is even more fascinating about the feathers, however, is the way they are structured.

Ordinarily most feathers are the same on the front and the back, but not the tail feathers of the woodcock. The birds are able to have both cryptic and conspicuous colouration on the same feather which changes depending at what angle it is being viewed from.

'Think of billboards you might see on the highway,' explains Alex. 'When you drive past it might be an advert for a car, but when you are a hundred yards down the road and you look back it is an advert for a restaurant. It is the same sort of thing.'

'So on one side of the venetian blinds the woodcock feather is a lovely brown, mottled forest for hiding, and on the other side it's the brightest feather known to exist.' 

The reason for the dazzling feathers is likely a result of the birds' habit of hanging around dimly lit forests. Under the gloom of the canopy, the sudden bright flash of white would be an incredibly efficient way to signal a bird's desire to mate.

This could imply that other woodland birds, such as owls and nightjars, might employ similar tactics. But after looking at these groups, the team couldn't find evidence that any have reversed feathers in the same way.

It appears that this is something that only the woodcocks are doing, and we don't yet know why.