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

Focus On...

the Scott's, Bullock's, and Hooded Orioles

An unexpected number of reports of Scott's Oriole brought this striking species to our attention this last winter. Twenty years ago there were very few winter records of the species in San Diego County. Since then, the Borrego Springs and Escondido Christmas bird counts have revealed it to winter regularly at least locally, and now our atlas effort is finally giving us a view of the surprising breadth of Scott's Oriole's distribution. The diversity of places where the Scott's Orioles have been found wintering, especially this past winter, has led me to a hypothesis: Scott's Oriole is not a migrant governed by the calendar in the same way as the Bullock's and Hooded Orioles, which arrive and depart almost like clockwork each year. Instead, I propose that the Scott's is a facultative or opportunistic migrant, more migrating in cold, wet winters, more staying in mild, dry winters like the last. Possibly, large-scale climate change is playing a role, warmer weather allowing the birds to winter farther north than previously. The historic winter range of Scott's Oriole is not far to the south of us—both A. W. Anthony and Laurence M. Huey found it in January and February near San Quintìn, just 150 miles south of Tijuana.

[Illustration of a Scott's Oriole]

Scott's Oriole

[Illustration of a Bullock's Oriole]

Bullock's Oriole

[Illustration of a Hooded Oriole]

Hooded Oriole

In the winter, Scott's Orioles seem to gather into flocks and wander in search of food. Several atlas participants—Sue and Jim Berg in Rancho Cuca (F14), Gail and Roger Wynn in Sherilton Valley (N19)—have noticed the orioles raiding the fruits of prickly pear cacti, then moving on once the supply of fruit is exhausted. Prickly pear thickets are the habitat for the Scott's Orioles found annually on the Escondido Christmas bird count on the grounds of the San Diego Wild Animal Park (J12, J13). They have been found since the count was inaugurated, as Ken Weaver points out, suggesting the species has long been a regular if rare and localized winter visitor. A predilection for prickly pears explains the late Eleanor Beemer's repeated observations of Scott's Oriole in Pauma Valley (E12), among the few pre-1980 winter reports from the coastal slope—thickets of prickly pear are prominent on the steep south-facing slopes at the base of Palomar Mountain. Another food source for wintering Scott's Orioles is the nectar of flowering eucalyptus trees, as I noted in north Jamul (R15) on 1 January 2000. In the Borrego Valley date palms are the prime habitat, with ornamental trees also frequented. During our desert blockbuster on 8 and 9 January 2000 we found the birds using native palm oases—Jim Wilson and Phil Nelson had one at Mountain Palm Springs (O26), Brennan Mulrooney had three at Bow Willow Palms, and Ann and Tom Keenan had one at Mortero Palms (S29).

Yet the orioles occur in winter in places where it seems there is little if any food for them at that season, nowhere more so than on the tops of the Santa Rosa Mountains, where during our January trip I found three males in C27 and Lori Hargrove found one in D28 at elevations over 5000 feet. Perhaps the males find it worth it to endure slim pickings for a few months for the sake of being first in line for reoccupying prime breeding habitat in the spring.

The atlas effort has improved our knowledge of the breeding distribution and ecology of Scott's Oriole enormously as well. Rather than inhabiting a general vegetation community or landscape type, the oriole focuses on a few species of plants that supply it with specific needs: yuccas and the Desert Agave. The Mojave Yucca or Spanish Dagger (Yucca schidigera) is the most important species, being the primary provider of the fibers with which the oriole builds its nest. I don't know if the orioles strip the curly fibers fringing the Mojave Yucca's leaves, in the same way that the Hooded Oriole strips fibers from a fan palm, but the old leaves of the yucca decompose into long tough but flexible fibers ideal for weaving. The Mojave Yucca also offers Scott's Oriole an ideal nesting site—Paul Jorgensen clued me into the bird's habit of attaching its nest beneath the crown of the tallest yucca in its territory. Nevertheless, Scott's Oriole may place its nest in a wide variety of other desert plants—juniper, smoketree, jojoba, catclaw acacia—placed as high in the shrub as possible while still taking advantage of the shrub's (often meager) ability to conceal it. The nests last for months, long after they have been used, so it is quite possible to confirm Scott's Oriole nesting on the basis of used nests even in the winter.

The flowers of the Desert Agave (Agave deserti) offer the Scott's Oriole its best feeding, presumably on both nectar and insects. Thus desert scrub in which both the yucca and agave are common constitutes ideal Scott's Oriole breeding habitat. I once thought of Scott's Oriole as restricted to the high-desert scrub along the steep east-facing slopes overlooking the Anza-Borrego Desert. Our atlas effort has shown the species to be much more widespread, nesting sparsely even on the desert floor where the plants favoring it are concentrated. Likewise, Scott's Oriole nests farther west, onto the coastal slope in arid chaparral, where the Mojave Yucca also grows. If Yucca schidigera is sparse or absent, a few birds on the edge of their range take advantage of the more montane Yucca whipplei.

The adult male Scott's Oriole in its stunning yellow and black dress is nearly unmistakable. Fortunately, the adult males are so conspicuous that when entering a Scott's Oriole territory you normally see—or hear—the male first. The song is remarkably similar to a Western Meadowlark's, with the flutelike notes more delicately phrased. Perhaps aware of their conspicuousness, the birds are shy, so scan the shrub tops at a distance before heading toward the source of the song.

The females and immature males, however, are comparatively drab, plus highly variable, creating opportunities for confusion. As in the corresponding plumages of the Hooded, the belly and undertail coverts are yellow, albeit a darker muted yellow. The back is more streaked with dusky than in the other orioles, sometimes strongly so. The greatest variation is in the extent of black on the head. In both immature males and adult females the black can range from practically none to covering the whole face and throat. Only the immature female lacks black on the face or throat consistently. In adult females, even if the head is entirely greenish, it is a dusky grayish green, often forming a shadow hood in the pattern of the male's.

The Hooded and Bullock's Orioles are far more familiar to most of us, but it may be worth reviewing the females' identifying features. The female Hooded Oriole is a more uniform paler, yellower greenish, lacking noticeable streaking on the back. Again, the belly and undertail coverts are entirely yellowish. Females lack a black bib, whereas immature males have one in the same pattern as the adult males. The Hooded's bill is the most notably curved of all our regular species of oriole.

Female Bullock's Orioles are distinguished by always having whitish bellies, becoming grayish on the flanks. Their undertail coverts may be slightly tinged yellow, but there is always wide separation between the yellow of the breast and that of the undertail coverts. The females vary greatly in possessing or lacking a small black bib, smaller than the Hooded's. Their white wingbars are broader than in the other species, echoing the large white wing patch of the adult males. The Bullock's Oriole's bill is absolutely straight, being shaped much like a Brewer's Blackbird's, and deeper at the base than a Hooded's or a Scott's, giving the head a noticeably different shape.

The San Diego County Bird Atlas has revealed interesting details of the distribution of the Hooded and Bullock's Orioles, too. The Hooded was an early adapter to urbanization, and the widespread planting of Washingtonia fan palms has favored them greatly. In the foothills, the uphill limit of the Hooded Oriole on the coastal slope appears to coincide with the planting of fan palms, the birds occurring above 1500 feet elevation only around homes where these have been used in landscaping. Closer to the coast, however, the Hooded uses other trees for nest placement, if not for nest material—under banana, fig, and dense clumps of eucalyptus leaves. In places it still uses its original nest site of sycamore trees in natural habitats, as in San Onofre Canyon in square D4 and along the Sweetwater River in R15.

Bullock's Oriole exemplifies an interesting pattern of distribution evident more strongly in several other species such as the Lark Sparrow and Lazuli Bunting. I might term it "anticoastal"—a distribution extensive over the coastal slope avoiding the coast itself. We think of the Hooded as the urban oriole par excellence but a surprising number of Bullock's nest within the city, especially where sycamores have been used in landscaping, as in Hilltop Park, Chula Vista (U11). Less frequent is the birds' nesting in urban eucalyptus trees, as on the grounds of the Educational Cultural Complex in southeast San Diego (S10).

While Scott's Oriole is now revealed as a regular winter visitor in the inland regions of San Diego County, it seems to avoid the coast—though this may be an artifact of the elimination of cactus thickets with urbanization. Bullock's Oriole, on the other hand, is a regular though rare winter visitor to urban parks and lowland riparian woodland.

Perhaps the biggest surprise of our Campo/Boulevard blockbuster weekend on 22 January 2000 was the Bullock's Oriole at the Crestwood Ranch in R24 by Ann & Tom Keenan and Jane Larson. There had been only winter record for San Diego County at so high an elevation, Mary Beth Stowe's on 16 January 1999 at Banner (K21). Yet it found another parallel this year, with Clark Mahrdt and Ed Hall discovering another Bullock's Oriole not far away near Campo in U22. Were these records symptoms of the mildness and dryness of the last two winters?

The Hooded Oriole, though the most familiar in the summer, is by far the least likely species to be seen in the winter—less likely, in fact, than the vagrant Orchard and Baltimore Orioles.

It's a pleasure that our three regular orioles, so delightful to see, can also teach us so much about adaptation to both natural and man-made environments.

-- Philip Unitt

For more about the Santa Rosa Mountains habitat,
please see Exploring the Santa Rosa Mountains

Oriole illustrations by Nicole Perretta

Focus On ... | Spring 2000 Wrenderings | Bird Atlas Introduction