Polymorphism: The White-throated Sparrow Story

Everyone knows the White-throated Sparrow. Ordinary, abundant, and widespread. Its call is unmistakable, nostalgic even. A gentle oh-sweet-canada-canada on a crisp Spring day.

What most people don't know is that this bird has a secret.

White-throated Sparrow by Matt MacGillivray (Flickr)

It exhibits a unique feature considered rare in birds. Polymorphism. In other words, the White-throated Sparrow may display two very different color forms or phenotypes—the observable characteristics and traits of an organism.

These phenotypes, commonly referred to as "morphs" can be observed in both sexes. The "white-stripe" morph typically has black and white stripes on the crown accompanied by bright yellow lores, characteristic of a text-book White-throated Sparrow. These individuals sing louder, are generally more territorial, and are more likely to breed with more than one partner.

White-throated Sparrow Tan-striped by Matt MacGillivray

The "tan-stripe" morph takes on a muddier appearance with black or brown and tan stripes on their crown and mottled yellow lores. Tan-stripe birds tend to practice different breeding strategies than their white-striped counterparts. They sing less, are much less territorial, and tend to provide better care to their young.

Observers often mistake these differences for sexual dimorphism, believing the colorful white-stripe morph to be male and the dull tan-stripe morph female, an error made even by some identification guides. In reality, the differences between a male and female White-throated Sparrow are much more subtle, nearly indistinguishable. 

Morphs, side by side. Photo by Hilton Pond Center

Interestingly, individuals almost always mate with a bird of the opposite morph. This union provides the best of both worlds for their young, with studies showing that dimorphic pairs often fair better in the wild than same-morph couples.

These discoveries just go to show that sometimes even a common species deserves a closer look. I imagine we can all learn a thing or two about avian behavior and breeding habits from the White-throated Sparrow. To learn more, check out the links below.

Hilton Pond Center - Spring Birds, Migrants, and Otherwise
A Closer Look at an Ordinary Species