We are familiar with soaring hawks overhead in search of prey. Red-tailed Hawks, Ferruginous Hawks, Swainson’s Hawks and others we distinguish by their markings, mainly their underside because that’s what we mostly see. But occasionally it gets a little confusing because one Red-tailed Hawk, for example, might be darker or lighter than the last one we saw. These variations in color are called phases or “morphs”, morphological variations, actually polymorphs, many colors. The question is “Why should there be such morphs?”
You may know the term “search image.” It’s typically applied to predators who learn a set of features of their prey such as color, markings, behavior, scent, sound, or some combination of some of those factors. Forming this mental search image speeds up the recognition of the prey and makes the predator more efficient. You probably have formed search images yourself. Let’s say you just bought a 1950 VW that you plan to restore and you figure these vehicles are very rare. But then you start to spot them on your travels, even though you may not recall having seen any before. They were there but you didn’t notice them until you created a search image for them in your mind.
This search image concept works in reverse; some prey species recognize their predators, at least intelligent prey like birds and mammals. Looking up, a mouse sees a dark shape soaring overhead and after a few missed passes by this dark shape, the mouse learns that this shape, the dark hawk, is dangerous and should be avoided. But later a light-colored shape shows up. The mouse is not concerned because this is not the dark shape it learned to avoid. Boom, no more mouse.
This prey-fooling mechanism works as long as one morph is abundant and the other relatively rare. When the rare morph, the light one in this case, becomes more abundant, as it will because it is more successful than the dark morph in duping and catching mice, the prey will develop a prey-avoidance image for the light hawk and the dark morph will become more successful. Thus, over some years, the two morphs will take turns being the successful one – the one the mice don’t avoid. And the same reasoning would apply to an intermediate form of the morph. That’s apparently the reason for the existence of one or more color phases, but I oversimplified the case. At the bottom of this blog are some references to scientific papers you can read that explain things in more detail.
There are morphs of other species, of course, like the Snow/Blue Goose, the Great Blue/White Heron, and the Rock Pigeon with its many forms. In these cases there is a alternate reason for the different plumages, but I’ll save that for another blog.
The Avoidance-Image Hypothesis and Color Polymorphism in Buteo hawks.Sievert Rohwer and Dennis R. Paulson. Ornis Scandinavica (Scandinavian Journal of Ornithology). Vol. 18, No. 4 (Nov., 1987), pp. 285-290
Morph specific foraging behavior by a polymorphic raptor under variable light conditions. Gareth Tate and Arjun Amar. Scientific Reports volume 7, Article number: 9161 (2017)
Colour polymorphism in birds: Causes and functions. P. Galeotti, P. Dunn, D. Rubolini, M. Fasola ,in Journal of Evolutionary Biology(4):635-46 · August 2003