Closer-Look-House-Sparrow-by-kimball-garrett-apr-1980

 Corrigendum: The captions for breeding season and non-breeding season male House Sparrows were reversed [in the above scanned image]. 

Closer-Look-House-Sparrow2-by-kimball-garrett-apr-1980

A CLOSER LOOK—House Sparrow

By Kimball Garrett

Originally published in The Western Tanager Vol. 46 No. 7 April 1980

Beckoning the average birdwatcher to take a closer look at the House Sparrow may initially seem about as reasonable as inviting a student of fine cuisine out for a Hostess Twinkie. After all, this most basic "trash bird" is a highly-disdained form of low-life which one must kick from underfoot on a stroll through the city, sensorially censor while searching for rare tidbits at desert ranchyards and oases, and shoo away from one's breakfast at any outdoor eatery. But, whereas some may argue that birdwatchers routinely see beauty in birds which ornithologists study with cold, scientific detachment, the House Sparrow has probably been looked-upon with more wonderment and fondess by ornithologists than by the ranks of birders. In its variation are demonstrated important ideas about molt, the hormonal bases for seasonal change in appearance, and the evolution of geographical variation.

The House (or English) Sparrow was introduced into northeastern North America in the middle of the nineteenth century, and colonized the remainder of the continent rapidly thereafter. Similar introductions on other continents have made this one of the world's most widespread and successful passerines. In southern California it occurs at virtually any outpost of human habitation, even within the higher mountains and arid deserts. Seasonal movement is poorly understood and probably not major, but there are scattered records of transients or vagrants away from known breeding localities (e.g. Santa Barbara Island).

Identifying the House Sparrow at the species level should present no problems; we've all had sufficient experience at this. In shape, plumage, voice, and behavior, it is quite distinct from our native sparrows. In fact the House "Sparrow" is not a true sparrow at all, but an Old World Ploceid or "weaver-finch." (One could similarly argue that our native sparrows are the ones that are not real sparrows but rather Emberizid "buntings," but we won't dwell on this.) Distinguishing the sexes of adult birds is also simple: The males are more richly colored and possess (to at least some degree) a black "bib." In the following paragraphs, then, I will touch on two subjects: The variation in plumage and bill color through the annual cycle in the male, and geographical variation in North America.

Some species of birds molt into an alternate (or "breeding") plumage prior to the nesting season. The House Sparrow exemplifies another common process. The black throat which is so prominent in the breeding season is not the result of a complete molt of throat feathers; instead, pale-tipped throat feathers which were renewed during the annual fall molt merely become worn over the winter, exposing the solid black feather bases (see figure). The shiny nuptial plumage of the starling is obtained in the same manner: The pale feather-tips (which cause the spangled winter appearance) wear down over the course of the winter. Breeding condition male House Sparrows are also recognizable by their jet-black bills; the change from the horn-colored bills of non-breeding birds is influenced by a seasonal increase in testosterone levels.

Are there different races of House Sparrows in California? (Or in North America in general?) For a bird that was introduced to the continent only a century and a quarter ago (from a rather homogeneous stock) the answer would seem to be an obvious no. But scientists who have studied the problem of House Sparrow variation in North America intensively for over two decades argue that within the relatively short time span since the 1850s, differentiation has matched that of recognized races of other species. Put simply, the House Sparrow has undergone evolution since its introduction into North America, to the point that populations from some areas are visibly distinguishable from those of other areas. But don't go rushing out to look for these differences. So far they have been arrived at by comparison of extensive series of collected specimens, by fine measurements of skeletal elements, and by sophisticated principal component analyses of such measurements. The differences (greatly oversimplified) include paler coloration in arid desert regions (e.g. Death Valley and Phoenix), larger size in cooler northern localities, and larger limb size in warmer regions. I have taken the liberty to dream up futuristic House Sparrows from Death Valley and Vancouver, in order to aid in visualizing these evolutionary trends (see figures). Please keep in mind that I stand on very shaky ground when trying to predict "outcomes" of evolution; I am simply assuming (for fun) that present evolutionary trends in House Sparrow variation in North America will continue, and spawn the creatures I have drawn. (In reality, a variety of ecological constraints would be likely to check these trends). The time scale involved cannot be predicted, but it should be noted that changes detected in the century since the introduction of this species to North America have been of a sufficient, magnitude to cause some re-thinking of the rates of evolution of higher vertebrates under "natural" conditions.

The evolutionary trends shown by North American House Sparrows roughly conform to three often-cited "Ecogeographic Rules." One, Bergmann's Rule, holds that body size tends to be larger in colder climates. Another, Allen's Rule, predicts larger extremities in warmer climates. Both of these "rules" presumably describe thermoregulatory adaptations (e.g. longer extremities, such as jackrabbit ears, more efficiently dump heat and therefore tend to be more common in warmer climates). Finally, Gloger's Rule predicts darker color in humid regions and paler color in hot, arid regions. Variation in many North American vertebrates corresponds to these predictions.

There may be more alluring species to watch than House Sparrows; but wherever birdwatchers roam, these birds can't be far away. And there is, therefore, a constant reminder of the power and (perhaps surprisingly) rapidity of natural selection in shaping the adaptations of this most successful trash bird.

Selected References

Johnston, R.F. and R.K. Selander. Science 144:548-550, 1964.

Evolution 25:1-18,1971.

Selander, R.K. and R.F. Johnston. Condor 69:216-258, 1967.


 

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