White-throated Sparrow Morphs

A White-striped White-throated Sparrow Zonotrichia albicollis

A Tan-striped White-throated Sparrow.

White-throated Sparrows are ubiquitous throughout their range in North America. Per this eBird range map you can see where it is most common.

There is one uncommon thing about the White-throated Sparrow. There are two color morphs. Some birds have tan colored head stripes with the black stripes between, and others have white stripes along with the black stripes. It took a while for ornithologists to figure out what this is all about. Thoughts might have been that one morph was male, the other female. That is not the case. It also is not range related or age related either.

It turns out that for some reason White-throated Sparrows are preferentially attracted to a bird of the other color morph to mate. There seems to be some benefit to a white-striped bird to mate with a tan-striped bird. When given the opportunity these sparrows preferentially breed with a bird of the other color variant. It was discovered in 1966 by Thorneycroft that white-striped individuals have a single M chromosome, and tan-striped birds lack this chromosome.
There has been a fabulous amount of research into how this stripe color difference plays into the lives of these sparrows. Ken Kauffman, one of the U.S. most prominent birders, wrote a wonderful article on thistopic in2017 https://www.audubon.org/news/the-fascinating-and-complicated-sex-lives-white-throated-sparrows
It appears that white-striped birds of both sexes are more agressive, and tan striped birds more nurturing. Tan-striped birds of both sexes are better providers for their young, bringing more food and providing more are. So you'd think that tan striped males would be more attractive to females than white-striped males. You are right in that regard, but because the white-striped females are more aggressive they snap up available tan-striped males quickly, leaving white-striped males for their tan-striped sisters.
This is one extremely complex mating system, and to me is really fascinating. So, next time you see a White-throated Sparrow check the color of its head stripes, and think about how this affects its lifestyle and mating behavior. Or if you like just marvel at the miracles of nature, and smile.

Until next time, good birding. Steem on.

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