Research may reveal one reason for color morphs, but there is much to learn.

In some birds, such as the well-known Red-tailed Hawk, the same species can occur in two or more color types, or morphs. New research may reveal at least one reason for this.

Scientists studied Black Sparrowhawks in South Africa. The species has both a dark and a light color morph. Research indicates that the hunting success measured by how much food they brought to their chicks differed depending on light levels. Thus dark birds did better when it was darker and light birds did better when it was brighter.

“Our study is the first study to reveal support for the idea that color polymorphism is due to different morphs being better adapted to different light conditions,” said Gareth Tate, PhD student at the Percy FitzPatrick Institute and lead author of the Ecology Letters study. “This is an important finding and helps evolutionary biologists understand how multiple color varieties can co-exist together in the face of natural selection.”

Dr. Arjun Amar, supervising author of the paper added, “We think that dark morph birds capture more prey in duller conditions because they are better camouflaged against darker cloudier skies. Within our study area, high rainfall coincides with when the species is breeding, and this may also explain why we have so many of this usually rare colour type here.”

In the United States the Red-tailed Hawk has highly variable plumage, including a very light phase, known as Krider’s Red-tailed Hawk and a very dark phase known as Harlan’s Hawk. Krider’s and Harlan’s were previously considered separate species. There is little in the literature suggesting reasons for the wide range of plumages.

 

Kriders morph - Red-tailed Hawk

Kriders morph – Red-tailed Hawk

 

Red-tailed Hawk

Typical Red-tailed Hawk

The common White-throated Sparrow has a subtle plumage morph that effects mate choice.  Individuals of the “white-striped” morph have primarily white and primarily black feathers in the median and lateral crown stripes. Individuals of the “tan-striped” morph have primarily tan and brown crown stripes.

The two forms are genetically determined. Individuals almost always mate with a bird of the opposite morph.

White-throated Sparrow with white eye line

White-throated Sparrow with white eye line

 

White-throated Sparrow with tan eye line

White-throated Sparrow with tan eye line

From the Bent Life History Series:
Investigations were made of behavioral differences between breeding adults of both types (of White-throated Sparrows) in Algonquin Park in order to determine at least some of the factors governing the assortative mating (Lowther, MS., and see also Voice). Experiments with tape recordings showed that white-striped males are more aggressive than tan-striped males toward singing individuals. Furthermore white-striped females sing, tan-striped females do not, and white-striped males act aggressively toward singing females, while tan-striped males do not. Finally, the trill note of a female elicits a copulatory excitation in males of both types, but when the trills are accompanied by songs of either males or females, this excitation of white-striped males is suppressed and is replaced by aggressive behavior. This is not true for tan-striped males, which were seen to copulate with their white-striped females, even when a tape recording of a strange male was being played.

While the research on the Black Sparrowhawk in South Africa may reveal one reason for color morphs of the same species, the situation is complex and much remains unknown.

2 Comments to “Color Morphs in birds”

  1. Bob says:

    I find your info re White-throated Sparrow (WTSP) contradictory. You say “Individuals almost always mate with a bird of the opposite morph”; this is consistent with the Birds of North America (BNA) species account for WTSP. Yet you also say that “males of both color types prefer females with white stripes” and “both types of females prefer tan-striped males.”

    Here’s what the BNA species account for WTSP says (along with a whole lot more detail):
    “a phenomenon that appears to be unique among birds. In brief, (1) the White-throated Sparrow is dimorphic in both plumage and behavior, corresponding to a clear genetic difference (chromosomal dimorphism); (2) this dimorphism originates from inversions of genetic material in the second chromosome resulting in many genes being inherited as a block or supergene, presumably including genes responsible for the dimorphism ( Thorneycroft 1966 , Thorneycroft 1976 ; Thomas et al. 2008 ) ; (3) individuals almost always mate with a bird of opposite morph, which promotes a stable dimorphism because TS (tan-striped) birds lack the rearrangement (chromosome 2) whereas WS (white-striped) birds possess it (chromosome 2m).”

  2. Sam Crowe says:

    Bob was correct in his comment. Correction has been made. Thanks, Bob.

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