Birds

Sexual Dimorphism in Birds

Sexual Dimorphism in Birds.

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Feathers set birds apart from any other animal on Earth. Plumage has a variety of important biological functions, including social signaling and survival.

Dimorphism

Differences in size, color, weight, and over all appearance are examples of dimorphism in birds. Birds that do not exhibit key differences in size, color, and over all appearance are considered monomorphic.

Monomorphism in Birds

An example of a monomorphic bird species is the American Crow (Corvus brachyrhynchos). These corivds look almost exactly alike. Mallards (Anas platyrynchus), however, look so different that they were once considered to be a different species of bird. Below are four examples of monomorphism in birds.

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Canada Geese. Image Credit: Animal Perspectives.

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American Crows. Image Credit: Animal Perspectives.

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Cedar Waxwing. Image Credit: Animal Perspectives.

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Chipping Sparrows. Image Credit: Animal Perspectives.

Sexual Dimorphism in Birds

Size and plumage dimorphism are both examples of sexual dimorphism. In species that are sexually dimorphic males might display more ornate and intensely colored plumage when compared to their female counterparts. Female plumage, in these examples, tend to be more drab, or cryptic.

Sexual dimorphism signals the sex of a bird to its species’ counterparts, it also plays a part in female mating preferences. During mating season males are, ideally, in optimal physical condition so that they can attract females. A brightly colored male is likely to attract a female looking for a genetically hearty mate — intense colors stand out to a female and signify to her that he’s a keeper.

The drabness of female plumage also plays a role in predator avoidance. A drab female is less likely to call attention to herself than a glamorous male while brooding a clutch, therefore, she might avoid detection. Below are examples of sexual dimorphism in birds.

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Female, Left, and Male (hybrids) Mallards. Image Credit: Animal Perspectives.

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Female Common Yellowthroat. Image Credit: Animal Perspectives.

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Male Common Yellowthroat. Image Credit: Animal Perspectives.

Female Tree Swallow. Image Credit: Animal Perspectives.

Female Tree Swallow. Image Credit: Animal Perspectives.

MALE TREE SWALLOW

Male Tree Swallow. Image Credit: Animal Perspectives.

Eastern Bluebird. Image Credit: Animal Perspectives.

Female Eastern Bluebird. Image Credit: Animal Perspectives.

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Male Eastern Bluebird. Image Credit: Animal Perspectives.

Size Dimorphism in Birds

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Female Northern Harrier. Males are Slate-Gray. Image Credit: Animal Perspectives.

Size dimorphism is often present in raptors. For example, in raptors, size dimorphism can be seen in the size difference between male and females, and sometimes plumage.

In general, size dimorphism is exhibited in a larger female when compared to a male, but that isn’t always the case.

Sometimes the differences in size are very subtle and only visible to the trained eye. Differences in size might not always be the best way to distinguish a male from a female. When this is the case a professional might take a sample and send it to a lab for analysis.

Test Your Skills

Sexual dimorphism can be obvious, however, it can also be subtle. For example, Northern Flickers (Colaptes auratus) are sexually dimorphic, but at first glance their differences might go unnoticed. Do you see how sexual dimorphism is presented in the two Northern Flickers in the photo below?

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Female and Male Northern Flickers. Image Credit: Animal Perspectives.

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