Tuesday, February 12, 2013

Hey! Look at me! Look at me!

Many species of animals exhibit a property called sexual dimorphism.  This means that there is a visible difference between the male and female of the species.  Examples in the world of mammals include large antlers on male deer while females usually have none, or manes on male lions but not on females.  In many species the male is larger than the female - this includes humans.

Some birds also show this difference in size between the sexes.  Sometimes the male is larger, like in Wild Turkeys, and in other cases the female is larger than the male.  This is common in raptor species.

The real show, when it comes to sexual dimorphism is birds, is in variations in color between the male and female.  In many species there is no visible difference, but in others the male and female are so different that they do not even appear to be the same species.  First, let's look at three species where the colors are the same for male and female but there is a difference in degree of brightness.

Male (left) and female (right) Eastern Bluebirds

The male and female Eastern Bluebird (Sialia sialis) both have the same color scheme: blue crown, wings, back, and tail; red chin and breast; white belly.  The male's colors are much more intense than the female's.  She looks faded compared to him.

Male (left) and female (right) Tree Swallows

Like the Eastern Bluebird, the male and female Tree Swallow (Tachycineta bicolor) are similar in appearance.  Both are white underneath and blue and green above.  The blue feathers on the male are very iridescent, reflecting shades of purple and green.  Older females do show some iridescence, but in general they appear duller and more green then the vibrant indigo of the male.

Male American Goldfinch

Female American Goldfinch

With the exception of the male's black cap, both the male and female American Goldfinch (Spinus tristis) have a similar pattern to their feathers.  However, during the breeding season, he is bright yellow. She is a duller yellow and olive green.  (During the fall and winter, males are duller and more closely resemble the female.)

Other birds, show a greater degree of variation between the male and female's appearance.

Male (left) and female (right) Mallards

The male Mallard (Anas platyrhynchos) has an iridescent green head, bright yellow beak, brown chest (not visible in this photo), gray body, and black rear.  The female is an overall mottled brown color.  Both the male and female have a bright blue patch on their wings, known as a speculum, that is most visible in flight.  

This pattern of a bright showy male and a mottled brown female is very common among ducks.

Male Rose-breasted Grosbeak

Female Rose-breasted Grosbeak

It is also common among songbirds. The male Rose-breasted Grosbeak (Pheucticus ludovicianus) is very distinctive with his black back, white belly, white wing patches, and rosy red breast.  The female is a mottled brown color and resembles a large sparrow.

Male Red-Winged Blackbird

Female Red-Winged Blackbird

Without knowing that they are opposite sexes of the same species, it would be easy to think that the male and female Red-wing Blackbirds (Agelaius phoeniceus) were separate species. The male is a glossy black with a bright red-orange and yellow patch on his shoulder.  The female again looks like a large sparrow with her mottled brown color scheme - she does have a red patch on her shoulder but it is rarely seen.

So it's obvious that many male birds are brighter colored than the females of the same species. They also behave differently.

Male Red-winged Blackbird singing and displaying

Singing male Tree Swallow - note the swollen throat

Male birds make themselves noticed.  They often find a high perch in their territory and sing.  Very few female birds sing.  Males sing both to attract mates and to show rivals how strong they are.  Essentially they are saying one of two things. First, "All of you other boys go away! This is my spot! Go away!".  Their second message is basically "Hey girls! Look at me! I'm the prettiest boy around! Look at me!"

Obviously, there must be an advantage for male birds to being brightly colored and to singing to make yourself known.  Brighter, louder birds are more likely to attract females and have more opportunities to mate than males who are duller colored and quieter.  

There is one big disadvantage to being noticed.  If female birds can notice you easier, predators will notice you easier too.  The brightest, loudest male birds are the ones most likely to draw the attention of predators.  So while they pass on their genes to their offspring, they themselves might be less likely to survive.

The females have found an advantage in being dull.  They blend in very well with their nesting materials.  A Red-winged Blackbird nest is made of dried grasses, cattails, and other plants.  It is often built in the dried growth of the previous year's cattails.  When the female settles into the nest she can be invisible from just a few feet away.  This ability to be camouflaged means that she is less visible to most predators.

As I tell students, even though the males tend to be prettier, I think the females are just a little bit smarter.

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