The idea that “false faces” are used by birds for predator deterrence floats in the ether between biology textbook case study and mythical meme. The idea that evolution can produce false faces compels and amazes us but because of the lack of solid evidence around the function of plumage patterns in many species we still drift back and forth along this spectrum. In some species, there can be little doubt that we are looking at legitimate false faces (see Greg Byron’s great post on Northern Pygmy Owls), but in many other species the picture is not as clear. Perhaps in these less-clear cases we are squinting too hard and seeing faces of ancient oracles imprinted on our toast. But then again, squinting can sometimes help us see things in a new way.
Literature suggests that false faces could evolve to deter predation in a number of ways including making the predator think it is facing the prey head on or that is has already been detected by the prey (both of which might reduce the likelihood of successful predation), or, by confusing the predator so that it does not know where the true face is (e.g., as in the American Kestrel).
David Sibley took on the topic of Downy Woodpecker false faces in a 2010 blog post. I don’t disagree with Sibley on the likelihood of false faces on Downys, but will admit that looking at the illustrations I could not see a false face – unless he meant that of an American Woodcock. I felt embarrassed about this; I thought that if there was a false face it must be staring me – and predators – in the face. I polled a random sample of others in the room (n=1) and found complete agreement with my view.
However, two sources of information helped me to think about this differently. First, a recent paradigm-shifting paper by Andrei Sourakov provided evidence to suggest that false heads on hairstreak butterflys serve not to deter predation by birds but by spiders: perhaps my focus is in the wrong place. Second, squinting at live Downys on my suet feeders made different patterns emerge relative to static photographs: movement is important to the face effect, and squinting, which serves to increase contrast, makes this effect come alive.
Here is some Downy footage with an illustration at the end to direct your eyes. The black vertical dorsal head stripe can appear as a dark beak; the eyes seem to be set into the black horizontal eyestripes on either side of the head; the black wings serve to frame the white back making it appear as the evil twin’s bright breast and belly. The face is most evident as the bird moves its head side to side. When the female Downy breaks from feeding and puts her bill forward the false face pop up as if to say “who’s dere b’y?!” [substitute expression appropriate to your corner of Canada].
This, I believe, is what is considered to be the classic Downy’s “false face”, yet the elements that seem to stand out the most are the white patches on either side of the vertical head stripe. When the bird moves her bill from a tilted-up to straight-ahead position, the white patches become quite eye-catching – and these elements would be most prominent under low-light conditions. Perhaps I am staring at the toast too long and there is no Screech Owl here, but then again, maybe there is:
And finally, an excavating Downy as a cavity-nesting duck (http://youtu.be/QugA875pUdw and http://youtu.be/kes5Isf4iDk):