By far, the most exciting bird activity in our yard for the past two weeks has been the small flock of fox sparrows that forages every day beneath our feeder near the back of the house. The feeder is adjacent to a fringe of brushy vegetation, something that attracts these birds. The sparrows alert us that they’ve arrived—they forage with a characteristic “double-scratch”—hopping forward, then immediately hopping backwards, raking their little feet through the leaf litter with such vigor that it is audible to a passerby.
These scritich-scratching visitors are big, brightly-colored sparrows. Field guides use adjectives “robust” and “chunky” to describe them. The only species in the Genus Passerella , their nearest kin seem to be the Zonotrichia sparrows—white-throated, white-crowned, golden-crowned and Harris Sparrows. You won’t find these birds in the new Minnesota breeding bird atlas, nor in the venerable Birds of Minnesota. Fox Sparrows don’t breed in Minnesota. They are just passing through, spring or fall. This visit, they are headed to breeding grounds in the far north bordering Hudson’s Bay. On the return trip (we usually have visitors both seasons) they will be heading to the southern United States.
Fox Sparrows are comprised of four distinct subspecies which nicely illustrates the process of the formation of new species by the geographic isolation of two (or more) populations. Geneticists postulate that prior to the last glaciation, Fox Sparrows lived from coast to coast in North America. Then, a glacier crept down a coastal mountain range, cutting off the eastern population from the coastal populations. Further isolation of three other groups resulted in four populations with distinctive plumages– the Red Fox Sparrow (Passerella iliaca unalaschcensis), the Slate-colored Fox Sparrow (P.i. schistacea); the Sooty Fox Sparrow (P.i. fulginosa) and the Thick-billed Fox Sparrow (P.i. steshensi). In Minnesota, our regular visitor is the Red Fox Sparrow, the most widely distributed of the groups. There are a number of intergrade plumages in the coastal populations and the American Ornithologists Union has been reluctant to grant the four Fox Sparrow groups species status. But analysis of mitochondrial DNA has shown the four groups to indeed be separate species. according to Dr. Bob Zink, former professor at the University of Minnesota. To go further into the weeds of fox sparrow speciation, see the Central Valley Bird Club’s publication, vol. 19 no. p.28 for a well-written technical paper by Steve Hampton.
Fox Sparrows on their breeding grounds in the far north, or along the Pacific coast, seek out brushy habitat similar to their winter preference, and nest on the ground or in low crotches of conifers. The far-north breeders do not have time to mess around. Within a day of arrival, say, from my backyard, they have established territories. Within a week, they have paired and begun nest building and egg laying. One brood only under the limited summer of the far-north.
Ornithologists at National Audubon have predicted that a warming of 3 degrees C. will bring with it wild fires, spring heat waves and heavy rains, all of which will be detrimental to a species that looks to a temperate far north as a breeding ground.
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