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Hiding at plain sight: tiny feathers help to conceal chicks

posted Mar 11, 2021, 3:59 AM by Clemens Kuepper

Snowy Plover chicks are adorable fluffy little critters. At least for us – we enjoy watching them bumble around at our study site. For others, these flufflings are more than food for the eyes, quite literally, perhaps the next snack – a few welcome extra calories that can become crucial in their daily struggle for survival. As the chicks can’t fly yet and most predators will outrun them, especially when they are young, they rely on camouflage to avoid detection by their foes.

What adaptations have chicks evolved to stand a chance in this game of hide and seek? This is a question of life and death and provides these tiny Snowy Plover chicks with an extraordinary challenge as they live on the salt flats where there are little places to hide. It’s no rocket science to assume that their feathers will provide some protection. The chicks’ mottled plumage, which features many colours of the soil, helps them to blend into the background. Yet, right now we actually only presume that this is the case - there is a dearth of studies on camouflage of chick plumage; not only for Snowy Plovers but pretty much for all precocial chicks.

The colours are not the only way how the chicks can use their feathers to evade predators. When you look closer at a plover chick, you will notice that there are tiny feathers sticking out from the chick in regular irregularity. This is a common theme for almost all chicks of shorebirds and other precocial birds. A very cool example for this is represented by Golden Plover chicks, which might be easily mistaken for a moss. Appendages are not restricted to birds alone, strange skin elements that stick out of the body are actually a common theme of a number of prey species. Prominent examples come from frogs, cuttle fish and of course, hairy caterpillars. Often these appendages have been heavily implicated in serving masquerade, another important camouflage mechanism where animals imitate in an inedible object. Yet, we wondered whether the tiny feathers actually help the small chicks to hide, and in particular, they serve to conceal the chicks outline when there is no place to hide.

Vroni Rohr took the lead for this study and investigated whether and how appendages such as neoptile chick feathers reduce detectability for her master thesis. She first tested the principle with an artificial object: a simple circular shape on a plain monochrome background to which she added ‘appendages, simple lines sticking out with varying density. In this process Vroni created a lot of digital images and measured contrast and luminance of the area surrounding the circular object with appropriate image analysis software. The first results were promising. She found that the appendages indeed reduced the contrast between the outline and the background and led to a transition zone of luminance. Both results were in the predicted direction suggesting that the appendages helped to make the outline of the object harder to detect. Over the course of her digital experiment, Vroni then varied various parameters of appendages and background to capture some real-world characteristics of feathers, habitat and also predator visual system. She found that the appendages actually had the largest effect when accounting for spatial acuity, a key property of the visual system of the predators. Spatial acuity is the ability to resolve two points in space, the higher it is the better the eye sight. Mammalian predators that are commonly eating plover eggs and chicks have a rather low acuity and here the results suggest that appendages made the biggest difference and helped to conceal the outline of the object the most.

In a second step, Vroni then took images from real Snowy Plover chicks in their natural hiding places, cropped them and then put them back at exactly the same place on a second picture that we had taken from the empty background and included the chosen hiding spot of the chick. However, before doing so, she digitally shaved off the tiny feathers in the treatment group whereas she transferred the chicks with their protruding feathers in the other group. These feathers were protruding only for a few millimeters from the chick and Vroni then compared contrast and luminance of the region around the chicks. Amazingly, the shaving treatment actually had an effect. Without the feathers sticking out, the outline of the chick had a higher contrast to the background suggesting that the shaved chicks would be more likely found. The shaving, however, made no difference for the luminance in the transition zone, so the concealment is more likely to work through reducing outline contrast.

Based on these results, we argue that this function of the chick feathers has so far been overlooked. The study was recently published in Scientific Reports. Now, whilst we are excited about these findings, it is important to put things into perspective and not get carried away too far. The better outline concealment alone will not be enough for the chick to escape the predators’ notice. There is no doubt that the main protection is most likely coming from the cryptic plumage colouration. (Interestingly, the camouflage of cryptic eggs has been studied alot more in detail already probably because it is an immense effort to locate precocial chicks and keep them still for the picture, as we learnt during our study.) Also, mammalian predators most likely rely on their noses to find the chicks although their eye sight might be important. Still every little helps in the race of survival and the protruding feathers contribute their little bit. We also noted that the juvenile shorebirds (and other baby birds!) keep these protruding feathers pretty much until the age when they can fly giving more credibility to our hypothesis that this is an adaptation to avoid detection by ground predators.

Clearly, chick camouflage deserves a lot more attention and future studies.