San Diego Natural History Museum--Your Nature Connection[San Diego County Bird Atlas Project]

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Drawings of heads of adult Violet-green and Tree Swallow, by Philip Unitt ADULTS
Tree Swallow (top)
Violet-green Swallow (bottom)
Violet-green and Tree Swallows

Among the species on which the San Diego County Bird Atlas has already generated interesting data are the Violet-green and Tree Swallows. These two species have similarities in both appearance and ecology, so it's worth comparing them. Adult males of the two species are easy to distinguish in a good view -- something not always possible with swallows flying high overhead, since both species have basically white underparts. The adult male Tree Swallow, with its deep uniform glossy blue underparts, differs obviously from the Violet-green, with its bronzy green crown, pure green back, violet upper tail coverts, and white patches on the sides of the rump.

The difference in pattern on the face -- eye surrounded by dark on the Tree Swallow, largely by white on the Violet-green -- is best seen when the birds are perched. Female Tree Swallows differ only slightly from the males, having variably duller upperparts and the breast faintly tinged gray. Female Violet-greens, on the other hand, are much duller than the males; their crowns can be almost plain gray, with hardly any bronze gloss, and the green and purple of the back and rump are less brilliant. Also, the cheeks of the female Violet-green are variably mottled with gray, making the two species' difference in face pattern far less obvious.

Drawings of heads of adult Violet-green and Tree Swallow, by Philip Unitt JUVENILES
Tree Swallow (top)
Violet-green Swallow (bottom)
In juvenile plumage the two species are more similar, with the upperparts brown in both with little or no gloss, rendering them similar also to the Rough-winged Swallow. The white patches on the sides of the rump of the juvenile Violet-green are still present if often less extensive than on the adult. The cheeks of the juvenile Violet-green are so heavily mottled with dusky that at any distance the face pattern of the two species looks the same. The juvenile Tree Swallow, however, has more or less of an ill-defined grayish breast band, contrasting with a pure white throat. The breast band is always much broader and fainter than the crisp narrow brown breast band of the Bank Swallow. The juvenile Violet-green has the throat lightly tinged gray, fading gradually to white on the belly, with no breast band.

Both species nest typically in holes in trees. The Purple Martin is the only other swallow in San Diego County to use this habitat. Rough-winged Swallows nest in holes in banks or bridges, while the Cliff and Barn build nests of mud. In addition, the Violet-green sometimes nests in rock crevices -- perhaps more often at lowland sites, where there are fewer trees, than in the mountains.

Here are our results with these two species so far, presented in form similar to how they will be presented in the final atlas, portraying the four levels of confirmed, probable, possible, and probably not breeding. To exclude migrants, the maps show only records from May, June, and July, as well as probable and confirmed nestings. The Violet-green, as is well known, nests primarily in pine/oak woodland in the mountains of San Diego County -- our data show clusters of records in the Palomar, Cuyamaca, and Laguna mountains. Less well known but shown clearly by our results is that the species probably breeds around Morena and El Capitan reservoirs --thanks to the efforts of Sue Smith and Rich and Susan Breisch at Morena, Steve Kingswood and Robert Sanger at El Capitan. Before this year, only a few scattered nestings were known at lower elevations, but several records this year suggest Violet-greens are more frequent in the lowlands than previously suspected. Confirmations of nesting in squares M14 by Julie Savary and J8 by Jim Zimmer show that these occurrences aren't just of migrants or nonbreeders.

The Tree Swallow, in contrast to the Violet-green, nests largely in the lowlands. San Diego County is at the southern limit of its breeding range, so it's not surprising the species is rare and scattered in summer -- though our results suggest it is more widespread than previously thought. The nesting pair found by Tony Mercieca at the north end of Lower Otay lake in square T13 is the southernmost nesting of the Tree Swallow yet known -- if only a few miles south of the next site, on the Sweetwater River at Highway 94, where the species has nested for several years and found by Don and Marjorie Hastings still probably nesting this year. Another notable Tree Swallow record, also by Tony Mercieca, is of a pair using a nest box in square J19, Wynola. The Violet-green Swallow would also be expected nesting in this mountain area. Jim Zimmer's observations in J8 show conclusively that the two species can nest near each other.

--Philip Unitt, from the winter 1997 issue of WRENDERINGS, drawings by Philip Unitt

Focus On ... | Winter 1997 Wrenderings | Bird Atlas Introduction