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

Yellow Warbler
Dendroica petechia

Photo of male yellow warbler by Anthony Mercieca
Male in spring migration at Chula Vista.
Photo by Anthony Mercieca.
Photo of female yellow warbler taken by Anthony Mercieca
Female in spring migration at Chula Vista.
Photo by Anthony Mercieca.

  Binocular graphic Good sites for seeing Yellow Warblers in San Diego County (April-August):
 
  • U.S. Forest Service's San Luis Rey Day Use Area along Highway 76 (G16)
  • Kit Carson Park, Escondido (J11/K11)
  • San Diego River, Mission Trails Regional Park (P11)
  • Mast Park, Santee (P12)
  • Sweetwater River at Highway 94 (R13)
  • Tijuana River Valley Regional Park (V11/W10/W11).
The Yellow Warbler symbolizes mature riparian woodland, that is, streamside cottonwood, willow, alder, and ash trees that have reached their full height. It is a fairly common breeding summer resident in this habitat, though the habitat itself is scarce and patchy. Though the Yellow Warbler is recognized by the California Department of Fish and Game as a species of special concern, since the late 1980s San Diego County's population has increased, evidently in response to the widespread trapping of the Brown-headed Cowbird, which parasitizes the warbler heavily. The Yellow Warbler is also common as a migrant passing through the county. It is rare as a winter visitor, in riparian woodland near the coast almost exclusively.


Adult male Yellow Warblers are easily identified from their chestnut streaks on bright yellow underparts. Females and immatures are more easily confused with two other largely yellow warblers, the Orange-crowned and Wilson's. The Yellow's blunt but tapered bill differs in shape from both the slender, needle-pointed bill of the Orange-crowned and the short, flattened bill of Wilson's. The Yellow has a vaguely paler eye ring but lacks the paler eyebrow of the Orange-crowned. The Yellow is distinguished in all plumages from other warblers by the broad brighter yellow edges on its wing coverts and tertials--easier to see in the field than the yellow spots on the inner webs of the tail feathers. Some juveniles have so little yellow as to look practically white, but the molt out of juvenile plumage starts soon after the young fledge. In fall migration, in October and November, beware also the immature females of the subspecies rubiginosa; these are so drab they look more a pale buff than yellow.

Breeding map for Yellow Warbler
                            Click for larger map
Breeding Season
Dark green: > 1 bird per hour
Light green: 0.5-1 bird per hour
Yellow: < 0.5 bird per hour
Double shading: Breeding confirmed (61 squares)
Single shading: Breeding probable (72squares)
Unshaded yellow or green: Breeding probable (84 squares)
Gray: Presumed migrants only

Breeding distribution: The Yellow Warbler's distribution is one of the more difficult to interpret, because migrants headed north may be seen through much of the season when the local population is nesting, and in the same habitat. Males sing freely in migration, negating that clue to territoriality. The interpretation of just what sightings to designate as in "suitable habitat" thus required judgment and review in the context of the entire data set. With the population expanding, some birds were pioneering into marginal habitat. Three late June records for the Anza-Borrego Desert mock any attempt to define a "safe date" after which no spring migrants are seen. The designations of breeding as "probable" and "possible" must be taken more literally for this species than for many others.

Despite these caveats, the Yellow Warbler's breeding distribution is clear: riparian corridors on the coastal slope. There is one area of known breeding on the desert slope, San Felipe Valley (J22; 50--probably including some migrants--on 21 May 1999, E. C. Hall; feeding young on 13 July 2001, P. Unitt). A singing male in a cottonwood grove at San Ignacio at the headwaters of Borrego Palm Canyon on 16 June 1999 (E22; P. Unitt) suggests breeding at that site.

In the coastal lowland, breeding Yellow Warblers are most widespread from Carlsbad north, more localized farther south. At low elevations the species is more confined to larger streams; in the foothills and mountains it takes advantage of narrow strips and patches of riparian trees. Surface water favors Yellow Warblers strongly but is probably not essential, as long as groundwater suffices to support tall trees. The Yellow Warbler's attachment to mature riparian woodland in southern California contrasts with its habitat in the more humid parts of its transcontinental range, where it inhabits lower thickets and disturbed and early successional habitats (Lowther et al. 1999).

Some sites where breeding Yellow Warblers are exceptionally numerous are the Santa Margarita River north of Fallbrook (C8; 64, including 60 singing males, on 24 May 2001, K. L. Weaver), the east end of Lake Hodges (K11; 50, including 40 singing males, on 18 April 1997, E. C. Hall), and the Tijuana River valley (W11; 40, including 30 singing males, on 27 June 1998, P. Unitt). Away from the main rivers numbers are much smaller. Still, San Diego County appears to be one of the main population centers for the Yellow Warbler in California, along with the Santa Ynez River in Santa Barbara County and the east base of the Sierra Nevada in Mono County (S. Heath pers. comm.).

Migration: Spring arrival of the local population of the Yellow Warbler is in March, typically in the last week, sometimes in the third week. One along the Sweetwater River near Jamacha (R14) on 8 March 1998 (M. and D. Hastings) was exceptionally early. Migrants headed farther north become frequent in mid-April, peak in May, and occur regularly through the first week of June. Numbers of spring migrants seen in a day at nonbreeding localities may run as high as 40, as in Vallecito Valley (M24) on 24 May 1999 (P. D. Jorgensen). Two at Tamarisk Grove (I24) on 16 June 1998 (P. D. Jorgensen), one in Borrego Springs (F24) on 20 June 1998 (M. Gabel), and another nearby in Borrego Springs (G24) on 21 June 1998 (P. D. Jorgensen) were very late stragglers, later than any spring Yellow Warbler recorded in the Salton Sink (Patten et al. 2002).

Winter distribution map of Yellow Warbler
                            Click for larger map
Winter
Dark blue: Maximum daily count 3-8
Light blue: Maximum daily count 1-2

Winter distribution: In winter the Yellow Warbler is a rare but annual visitor, mainly in riparian willows, also in ornamental plantings. Usually only a single individual is seen at a time, but multiple birds are regular in the Tijuana River valley, up to 8 around the Dairy Mart pond (V11) on 19 December 1998 (G. McCaskie). The birds can survive the winter successfully and even return to the same spot in successive years, as one has done to Myoporum trees at Famosa Slough (R8). Almost all records are from low elevations near the coast, inland to Valley Center (G11) and Lindo Lake (P14), with one notable exception, one near the navy's La Posta Microwave Station (T23), elevation about 3000 feet, on 21 February 1998 (C.R. Mahrdt).

Nesting: Yellow Warblers build a cup nest, placing it typically in upright forks of twigs. One nest in Peutz Canyon (P16) was along the trunk of an alder tree, supported by a slab of loose bark (M. B. Stowe, P. Unitt). The two nests whose height our observers estimated were about 23 and 35 feet above ground-well above the average height reported by studies elsewhere (Lowther et al. 1999).

Graph of Yellow Warbler nesting
                               Click for larger graph
        NB = nest building       NN = nest with nestlings
        ON = occupied nest     FY = feeding young
        NE = nest with eggs     FL = fledglings

Though many Yellow Warblers arrive in March, apparently they do not begin nesting until well into April. Our dates of breeding activity are consistent with dates of eggs collected 1903-1931: 3 May-10 June (n = 20); Sharp (1907) reported 20 June. The nesting schedule implied by our observations allows ample time for the birds to raise two broods. Previous studies (Goossen and Sealy 1982, Lowther et al. 1999) found the Yellow Warbler only rarely attempting two broods, but these studies were made at latitudes far to the north of San Diego. Indeed, virtually all of what has been published on the Yellow Warbler's biology comes from regions remote from southern California.

Conservation: The Yellow Warbler is well known throughout its range as a frequent host of the Brown-headed Cowbird-and famous for its response of flooring over parasitized nests to build a new nest atop the old. Nevertheless, many parasitized Yellow Warblers end by raising cowbirds anyway. Like that of many other riparian songbirds, the population of the Yellow Warbler in southern California collapsed during the mid-20th century under the double onslaught of the cowbird invasion and the elimination of riparian woodland. Then, once the Least Bell's Vireo was formally listed as endangered in 1986, cowbird trapping began at many sites throughout the county, and the Yellow Warbler was among the species whose numbers resurged. Unfortunately, rigorous numerical data with which these changes in Yellow Warbler abundance could be assessed are lacking, and habitat changes, like the regrowth of riparian woodland in the Tijuana River valley, have played a role too. In 1984, I called the species only "uncommon" as a summer resident, and counts of dozens in a day along a two- or three-mile strip of river, as found at the most favorable localities now, were unknown. The species has refilled apparently all of the San Diego County range from which it retracted before 1980. From 1997 through 2002, we recorded only a single instance of cowbird parasitism on the Yellow Warbler-a female feeding a fledgling cowbird in Kit Carson Park (J11) on 24 June 1998 (W. Pray). Beyond the cowbird trapping, regulations restricting the removal of riparian woodland and channelizing of streams have been critical in slowing the loss of the Yellow Warbler's habitat. Also, the damming of rivers has largely eliminated the flooding that once knocked over large trees, allowing more woodland to mature to the point where it attracts Yellow Warblers. Continuing negative factors, though, are the proliferation of the exotic giant reed Arundo donax, which replaces native riparian trees, and the pumping of groundwater, which lowers the water table to the point where these trees can no longer survive.

Taxonomy: The Yellow Warblers nesting in San Diego County, and most migrants as well, are D. p. morcomi Coale, 1887. The few Rocky Mountain specimens I have seen do not differ consistently from California specimens, dissuading me from following Browning (1994) in resurrecting brewsteri Grinnell, 1903, for the California population (Patten et al. 2002). Spring males have a yellow forehead contrasting with the greenish remainder of the upperparts. Each age and sex class considered separately, morcomi is brighter yellow than rubiginosa Pallas, 1811, the darker subspecies breeding along the Pacific coasts of Alaska and British Columbia. Spring males of rubiginosa have the entire crown green down to the base of the bill. D. p. rubiginosa migrates through San Diego County in both spring and fall. Its spring migration is concentrated in the second half of May (7 of 9 spring SDNHM specimens), though records extend from 7 April to 1 June (Unitt 1984). Four fall specimens are from 8 to 15 October, a late one from 21 November, part of the pattern suggesting that rubiginosa is a late migrant in fall (Patten et al. 2002). Though no winter specimens have been collected, most or all of the Yellow Warblers occurring in winter are bright yellow, implying morcomi.

Literature Cited

Browning, M. R. 1994. A taxonomic review of Dendroica petechia (Yellow Warbler; Aves; Parulinae). Proc. Biol. Soc. Wash. 107:27-51.

Goossen, J. P., and Sealy, S. G. 1982. Production of young in a dense nesting population of Yellow Warblers, Dendroica petechia, in Manitoba. Can. Field-Nat. 96:189-199.

Lowther, P. E., Celada, C., Klein, N. K., Rimmer, C. C., and Spector, D. A. 1999. Yellow Warbler, in The Birds of North America (A. Poole and F. Gill, eds.), no. 454. Birds N. Am., Philadelphia.

Patten, M. A., McCaskie, G., and Unitt, P. 2002. Birds of the Salton Sea: Status, Biogeography, and Ecology. Univ. Calif. Press, Berkeley.

Sharp, C. S. 1907. The breeding birds of Escondido. Condor 9:84-91.

Unitt, P. 1984. The birds of San Diego County. San Diego Soc. Nat. Hist. Memoir 13.

Contact: Phil Unitt
Phone: 619.255.0235; Fax: 619.232.0248; Email: birds@sdnhm.org
Bird Atlas, San Diego Natural History Museum, P.O. Box 121390, San Diego, CA 92112-1390



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