Focus On...Plumbeous, Cassin's, and Gray Vireos
The recent decision by the American Ornithologists' Union Checklist Committee to regard the Solitary Vireos as three species (Cassin's, Plumbeous, and Blue-headed) rather than one has raised the profile of these birds among many birdwatchers. Given that all three have occurred in San Diego County (the Blue-headed as a rare vagrant only), their identification inevitably interests us. The Gray Vireo also is easily confused with them, by song as well as by sight, suggesting it be can be addressed usefully in the same article.
Each of the components of the Solitary Vireo complex has a distinct breeding range, as implied in their former ranking as subspecies. Cassin's Vireo nests in forests in and west of the Cascade Range and Sierra Nevada, extending east into Idaho and western Montana. Toward the south it becomes more and more restricted to higher elevations, in Baja California breeding only in the Sierra San Pedro Mártir and Sierra de La Laguna. The Plumbeous Vireo nests from the Great Basin and central Rocky Mountains south through the Sierra Madre Occidental of mainland Mexico. Historically its breeding range was separate from those of the Cassin's and Blue-headed, but 40 or 50 years ago the Plumbeous began spreading west and northwest, coming into contact with Cassin's in the San Bernardino Mountains and on the east flank of the southern Sierra Nevada. Though adults of the two forms have paired, no hybrids are known, contributing toward the recognition of the two as species rather than subspecies.
The main difference between the Plumbeous and Cassin's is in back and flank color, Cassin's having an olive-tinged back contrasting with the gray crown and extensive yellowish along the sides and flanks. The Plumbeous has only the faintest hint of olive on the rump, in fresh plumage only; the back is the same gray as the crown. Similarly, its flanks are grayish with only the faintest hint of yellowish posteriorly. The head of a Plumbeous Vireo is deeper gray than that of Cassin's, bringing the white "spectacles" into bolder contrast. The "spectacles" of Cassin's are obvious, but the greater contrast of Plumbeous gives that species an almost startling bug-eyed look. The white throat of Cassin's blends gradually into the gray sides of the head; the white throat of the Plumbeous stands out much more sharply. Likewise, the wingbars and edges of the wing and tail feathers stand out in greater contrast on the Plumbeous because the dark areas are darker. The edges are pure white on the Plumbeous but mostly yellow-olive on Cassin's. The Plumbeous is a bigger bird than the Cassin's in all dimensions, by about 15%--not enough to be assessed conclusively in the field, but enough to contribute to the Plumbeous' being a more imposing bird.
In San Diego County Cassin's Vireo may be seen at all seasons but is uncommon during migration and rare both in the breeding season and winter. In summer it occurs in the high mountains, above 4000 feet elevation, in mixed woodland of oaks and conifers. Our atlas data so far show it in the Palomar, Cuyamaca, and Laguna mountains but very sparsely. After three years we still have only 23 records during the breeding season, and only one of these is of more than two individuals: Ed Hall and Jim Zimmer noted five in the Wooded Hill area of the Laguna Mountains (square P23) on 3 June 1999.
Cassin's Vireo was apparently never abundant in San Diego County but was formerly more widespread and common than currently. Presumably it fell victim, like so many other vireos, to the invasion of the Brown-headed Cowbird. In 1919, just as the cowbird was beginning its population explosion, Frank Stephens called the vireo a "rather common summer resident of timbered cañons in the mountains." Egg collections up to 1933 attest to Cassin's Vireo's nesting at somewhat lower elevations than currently, at Witch Creek, Descanso, Campo, and even Lake Hodges.
With several years of widespread cowbird trapping in San Diego County now behind us, might Cassin's Vireo recoup some of its losses, as Bell's Vireo has done so dramatically? So far, I know of only one record to suggest this. On our Valley Center blockbuster weekend, 6 June 1998, I heard a leisurely, phrased song in oak woodland along Lilac Road near Keys Creek (F11). Despite 20 minutes of waiting and searching I could never see the bird, which finally fell silent. I feel virtually certain it was a Cassin's Vireo. My only doubt arises from my observation of Purple Finches nearby--the Purple Finch sometimes sings a phrased song amazingly similar to Cassin's Vireo's, but in 20 minutes it should have shifted to its typical even warble a few times.
In sharp contrast to the decline of Cassin's Vireo, the career of the Plumbeous has been one of expansion. The species was first recorded in California in 1960, in San Diego County in 1969. It occurs as a winter visitor and migrant only -- so far. With its colonizing the San Bernardino and San Gabriel mountains, eventual colonizing of San Diego County's mountains seems possible. Over the past 30 years, the Plumbeous has become gradually more frequent until today it is more frequent as a winter visitor than Cassin's: in the winter data base from the first two years of our atlas effort records of the Plumbeous outnumber those of Cassin's by 12 to 8.
The usual winter habitat of both species is the same: riparian woodland in the coastal lowland. Most of our atlas records for the last two winters are from such situations: Cassin's has been noted in Kit Carson Park (J11, Wayne Pray), lower Los Peñasquitos Canyon (N8, Don Adams), and along the San Diego River near the mission (Q10, Joe Barth); the Plumbeous at Guajome Lake (G7, Pete Ginsburg), along San Marcos Creek near Rancho Santa Fe Road (J8, Jim Zimmer), and in the San Pasqual Valley (J12, Phil Unitt). The greatest concentration of both species has been around the pond at Dairy Mart Road and Servando Ave. in Tijuana River valley (V11), where Guy McCaskie recorded three Plumbeous on 19 December 1998 and two Cassin's one week later. Some wintering birds, as in the Tijuana River and San Pasqual valleys, return to the same spot in consecutive years.
Both species occur more rarely in other habitats--oaks and ornamental pines. Nola Lamken found a winter Cassin's at an elevation a little higher than typical at Silverwood Wildlife Sanctuary (N15) on 20 January 1998. Joanne Moore observed one Plumbeous in planted trees in the Mount La Jolla residential complex (P8) on 9 December 1998. Farthest afield was the Plumbeous discovered by Paul Jorgensen in pine trees on the grounds of the Roadrunner Club in Borrego Springs (F24) and followed through the winter of 1998-1999 by Bob Thériault and Mel Gabel--apparently just the third winter record of the Plumbeous Vireo for the Anza-Borrego Desert.
Occasionally I hear reports of the Gray Vireo away from its breeding habitat--arid chaparral at elevations of 3000 to 5000 feet--but I remain suspicious that most or all of these are actually of Plumbeous or Bell's Vireos. The Gray Vireo resembles the Plumbeous in its plain gray-and-white plumage, and its song is quite similar to Cassin's: burry phrases of one or two notes, separated by pauses so deliberate the bird gives the impression that it is trying to conserve its energy to survive the midday heat--often intense in the treeless chaparral where the Gray Vireo lives.
Close study shows many differences between the Gray and the Plumbeous. The Gray is smaller-bodied than Cassin's Vireo but its tail is longer than even the Plumbeous'; the Gray's bill is shorter than Cassin's but thicker even than the Plumbeous'. The lower wingbar is faint and the upper is nonexistent. The eye ring is faint and the white stripe through the lores, so conspicuous in the Plumbeous and Cassin's, is faint or nonexistent in the Gray. The bird's drabness is compounded by the fact that in San Diego County we see the bird only in worn plumage. The species molts just once a year, in its winter range (central and southern Baja California, Sonora), immediately after arriving in October--as I observed during the Museum's multidisciplinary expedition to central Baja in October 1997. So the birds' plumage is already six months old when they return in the spring, and the lack of shade and rough vegetation in their nesting habitat rapidly degrade it further. The end product is that the Gray Vireo is one of the drabbest birds in San Diego County.
Yet its scarcity, restriction to a remote and difficult habitat, and interesting history lend it a mystique. Its virtually never being seen in migration only heightens the aura of mystery. The Gray Vireo is now the rarest chaparral bird of southern California but a century ago--before the Brown-headed Cowbird invaded--it was apparently more common, though few ornithologists visited its habitat. Pioneer naturalist Frank Stephens, one of the San Diego Natural History Museum's founders, discovered the species in California and reported it from Oak Grove, Julian, and Campo. But from 1908, date of Stephens' last record, 69 years passed during which the Gray Vireo became a lost species in San Diego County. Finally, while doing a breeding-bird survey in 1977, Mike Evans rediscovered it in a small region between the Laguna Mountains and Interstate 8 (squares R22, Q23, R23, and R24), and it has been seen regularly in this area since, especially along Kitchen Creek Road. In 1984 a few resurfaced along the Indian Flats Road north of Warner Springs (squares D19 and E19), and scattered individuals or pairs have turned up elsewhere in mountain chaparral or desert-edge scrub. But our atlas effort is giving us the first clear picture of the Gray Vireo's distribution and status in San Diego County.
Jim Wilson and Lori Hargrove have been our champion Gray Vireo finders. Jim writes in his report from square Q22, Glen Cliff, "The best part of this square has to be the Gray Vireo. Before the Atlas I had seen the bird only once. When I saw the bird next on Cliff Mountain it did not look at all like a vireo but more like a large gnatcatcher with a long active tail, and, on some occasions, especially when singing, it would sit vertically, looking like a flycatcher. The old National Geographic field guide shows it to be similar to the Bell's Vireo, but the new edition's illustration is more realistic. I was never able to see any wing bars and generally it appeared quite plain except for an eye ring. I was never able to see any white on the lores. The ones I saw well were singing constantly a double note and a pause resembling something between a Black-headed Grosbeak and a Solitary Vireo. One male came near me next to the road singing away, making me aware of some activity even closer. Here was a female building a nest in a small scrub oak actually hanging over the road. In all I found three nests, one being built, one with an egg, and one with two nestlings. The best area was right where the hang gliders take off from Cliff Mountain and about one quarter of a mile north and south of there. The only problem is that you need a 4WD to get there or take a long uphill walk." Jim recorded a maximum of 11 Gray Vireos in this area, including 8 singing males, on 10 June 1999.
Lori writes, "The Gray Vireos are most easily heard than seen, and part of the difficulty in spotting them is that they are always farther away than they sound. When you do get a distant glimpse, often you see only a small plain bird, but one of the most distinctive clues is their paleness--an almost off-white gray. Once you've found a territory, however, with a lot of patience you can get quite close as the birds seem oblivious to your presence. At close range, the very fine eye ring is surprisingly noticeable."
Our atlas effort, along with a study sponsored by the U.S. Forest Service comparing birds' use of burned and unburned chaparral, have revealed the population discovered by Mike Evans to be slightly more widespread. It extends north along the lower slopes of the Laguna Mountains through squares Q22, P22, and O22 to upper Pine Valley Creek on the east side of O21. But the birds are only few and scattered in the last three squares. Likewise, a few extend south across Interstate 8 in squares S23, S22, and T21 on the south side of Lake Morena, where Susan and Richard Breisch made their high count of three individuals on 5 July 1997.
The Indian Flats population, evidently smaller than the Laguna one, extends west to Previtt Canyon along Chihuahua Valley Road (D17), where Andy Mauro heard one during our blockbuster weekend on 1 May 1999, and across Highway 79 to Aguanga Ridge, where Joe Barth and Bryan O'Leary found one on the same day in square E17. Ken Weaver encountered three farther west in D15 on 14 May, but Geoff Rogers' search along the Palomar Divide Road did not reveal any, underscoring the species' rarity.
Some new sites that have cropped up are along the Pacific Crest Trail northeast of San Felipe in square H20, where Claude Edwards had two on 24 May 1998 and Ann and Tom Keenan found one on 24 April 1999. Lori Hargrove discovered an unexpected pocket of five singing males along Goudie Road in O18 on 6 June 1999, giving some context to Kirsten Winter's observation near El Capitan Reservoir (O16) on 21 April 1998. To the east, Lori found a single bird in Lost Valley (R26) on 11 May 1999, and in the extreme southeast corner of the county (U29), Jim Wilson encountered another on 27 April, possibly the northernmost outpost of a population in the Sierra Juárez, where the Gray Vireo persists in substantial numbers.
Undoubtedly the two remaining breeding seasons of our atlas effort will reveal a few more sites. But the species' rarity and strange patchiness despite broad expanses of seemingly suitable undisturbed habitat is being reaffirmed.
Though it winters at least irregularly in the Kofa Mountains just east of the Colorado River, and north at least to the Sierra San Francisco in central Baja California, the Gray Vireo has never been adequately confirmed as wintering in Upper California. Along the coast of Sonora John Bates, now of the Field Museum in Chicago, observed them wintering in stands of the elephant tree Bursera microphylla and feeding primarily on the elephant tree's fruits--the first information on the Gray Vireo's winter ecology. On her long hike into Starfish Cove in square K28, Lori Hargrove encountered an extensive stand of elephant trees, perhaps the largest stand in California. Could these trees support an undiscovered wintering population of the Gray Vireo? We hope to make an expedition this winter to see. As a minimum 10-mile round trip it may be done best by overnight backpacking, requiring the carrying of extra water. Any others interested in taking up this challenge? Please contact me!
Philip Unitt, from the fall 1999 issue of WRENDERINGS