Avian Color Morphs

Swainson’s Hawk Light Morph

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.

Swainson’s Hawk Dark Morph

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

 

10 thoughts on “Avian Color Morphs

  1. Hello Mr. Lederer. Did you get to published more blogs about the reasons of color morphs on birds? I am writing from Costa Rica. Thank you for your teaching. Have a great week,

    1. Supposedly the reason for color morphs in Red tail Hawks and other raptors is to throw off the searching image of the prey (amall mammals), if we can give prey a searching image. Insects, who knows?

  2. I have a red morph Easter screech owl living in a owl box in my front yard. I do not know the color of its mate. I just got a first glimpse of one of the babies and it is gray! Would this mean its mate is gray or could they both be red and have a gray baby?

  3. The example you give about Screech Owls suggests that they are either red or grey,; no ‘blending’. But in hawks, Red-tails for example, that is not the case. There are many intermediate plumaged birds. Does this mean that the term ‘morph’ as it pertain to raptors and the term as it pertains to Screech Owls is different? Why do we get intermediate raptors but no intermediate owls?

    1. It all depends on genetics and physiology. In true morphs, one color or the other is determined by genes. In Red-tails, and lots of other birds, it is the deposition of pigment, often melanin, that deptermines the color – a lot like skin color of people of a particular race. Or dogs in a litter.

      1. Perhaps you have answered my question by using the term “true” morphs. Am I to assume that colour variants in hawks are not really ‘morphs’? Maybe we are using the term a little too loosely? My interest in this term was prompted by the use of the term ‘green morph’ to describe some Pine Siskins. I’m thinking that this, also, is not a true morph, but a pigment issue. Which would account for the variation in the extent of the green in different individuals. The question then becomes, how much green is required to describe a bird as a ‘green morph’? Or maybe the question is, “is there really such a thing as a ‘green morph’ Pine Siskin?”
        Is this green passed along genetically? Or is it a genetic mutation? Are green adults more likely to have green offspring? Do ‘dark morph’ Red-tails produce dark morph offspring? And in “true morphs”, for example, do ‘red’ Eastern Screech Owls produce ‘red’ offspring?
        I hope you don’t mind all these questions!!
        Thanks,
        Gary Davidson

        1. A morph is not a strict term. A morph is just a physical difference. So you could call the color variations in hawks morphs but more commonly the term morph is used to differentiate 2 or 3 alternatives. Pine Siskins have so called Green morphs but what the source of the green is I don’t know. Part of it is genetic because only males show the green morph version. But any variation in color could be natural variation, genetic, an aberration in pigment deposition, incomplete albinism, the food source, etc. You can find more in the scientific literature on Google scholar. Yes, red screech owls have red offspring but both gray and red morphs will interbreed.
          I think too much credence is given to color variation. It’s a natural part of evolution and may be interesting but usually not significant until it leads to the separation of populations and speciation. But birdwatchers like to point out any little difference.
          Nope, don’t at all mind questions. That’s why I have my website and blog.

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