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

Focus On...

Western and Cassin's Kingbirds

With a full year of both breeding and winter data in our files, and the capacity to analyze them effectively by computer, we now have a chance to look at our results much as they will appear in the final atlas. (Updated maps to appear here soon.) Let's use these resources to look at two interesting species, the Western and Cassin's Kingbirds.

Both kingbirds are conspicuous birds, using similar habitats: open country with scattered trees. Both build shallow cup nests, often fairly easy to find in crotches in the upper branches of open-canopied trees like sycamore, eucalyptus, and black locust. Both are noisy, the Cassin's repeating a characteristic loud "chi-beer," the Western irregular rapid "kik" notes. Both have olive-gray upperparts (darker olive in Cassin's, paler gray in the Western), gray breasts (darker with a contrasting white throat in Cassin's, paler and uniform in the Western), and yellow bellies. When the birds are in fresh plumage (as when they arrive in the spring), the pattern of the tail is perhaps the greatest difference in plumage between them: black with a sharp white outer edge in the Western, dark gray with softly blended paler edges and tip in Cassin's. In late summer, rely more on their voices and head pattern -- the edges of the tail may wear of entirely!

Western Kingbird, drawing by Philip Unitt Western Kingbird

How do such superficially similar birds differ in their biology and distribution? The Western Kingbird is a summer visitor only to California, so we have only one map for it, for the breeding season. The species is widespread, breeding in all regions of the county. Habitat suitable for Western Kingbirds, however, is far from uniformly distributed. Even when our coverage is more complete, the map for the Western Kingbird will likely look patchy, with the species lacking from squares covered with chaparral and woodland with no grassland or agriculture. In the Anza-Borrego Desert, the Western evidently breeds in places with tall trees (usually not native), such as in the Borrego Valley, near Ocotillo Wells, and in Mason Valley (Butterfield Ranch): But observations in most squares are of birds thought to be just passing through I migration, shown on the map with vertical lines. The Western Kingbird nests at higher elevations too, with confirmations near Lake Cuyamaca, and Pine Valley, for example. It seems to be most frequent I the inland valleys and tends to avoid the coastal strip. I've been surprised, however, by how many Western Kingbirds have been found nesting close to the coast, more than I expected. I never thought of the Western Kingbird as an urban bird, yet we have quite a few observations, including several confirmations of nesting in central San Diego.

Cassin's Kingbird, drawing by Philip Unitt Cassin's Kingbird

Cassin's Kingbird, by contrast, is found year-round in San Diego County. Though the species is migratory to some extent, the breeding and winter distributions look quite similar. Cassin's lives exclusively on the coastal side of the mountains and is absent from the Anza-Borrego Desert and the higher elevations. It is as widespread along the coast as in the inland valleys, which it shares with the Western. The inland edge of its range looks quite distinct, according to our data so far: Pauma Valley, Ramona, El Capitan Reservoir, Alpine, and Jamul. But a few interesting exceptions to this are evident, too. A population isolated from others in San Diego County (but presumably continuous from Riverside County) is on the east side of Palomar Mountain in C15, Dameron Valley, known as a site for many lowland (and desert) species through the discoveries of Ken Weaver. Likewise, a winter observation at Tecate during our blockbuster weekend by Dave Seals, Jim Wilson, and Frank Unmack suggest that the species extends north from Mexico into square V19, isolated from others in San Diego County by Otay and Tecate mountains. Cassin's Kingbird also occurs farther east than previously known in the Lake Morena area, where revealed by the observations of Sue Smith and Rich and Susan Breisch. Another isolated record is from Miller Valley I square S25, where found during one of our winter blockbusters by Margaret and Bert McIntosh. Miller Valley resembles Dameron Valley in lying on the coastal side but supporting certain plants and animals (Cactus Wren, Ladder-backed Woodpecker, White-tailed Antelope Squirrel) typical of the deserts. The microclimate may be on that suits Cassin's Kingbirds, too.

--Philip Unitt, from the summer 1998 issue of WRENDERINGS, drawings of Western Kingbird (top) and Cassin's Kingbird (bottom) by Philip Unitt

Focus On ... | Summer 1998 Wrenderings | Bird Atlas Introduction