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

Reports from the Field
Summer 2000

Brown-headed Cowbird Banding Project
The Last Holdouts
The Helicopter Trip to the Santa Rosa Mountains—Take 3

Brown-headed Cowbird Banding Project

The Brown-headed Cowbird is an obligate brood parasite, laying its eggs in the nests of other bird species and depending on the host to incubate its eggs and rear its young. Originally restricted to the central regions of North America, the Brown-headed Cowbird expanded in both range and abundance following the settlement and alteration of natural habitats, particularly with the increase in agriculture and livestock production. Reaching California in the late 1800s, this species was first documented breeding in San Diego County in 1915 and was well established in southern California by the 1930s. Songbird species or populations that did not evolve with the cowbird and had no experience with parasitism may suffer significantly reduced reproductive success. When combined with other factors, such as habitat loss and fragmentation, brood parasitism can lead to declines in host species, particularly those with an already limited population and distribution. Cowbird control through trapping programs during the breeding season has become an ever-increasing tool in conservation efforts for sensitive songbird populations throughout the United States. Initially applied in the recovery efforts for the Kirtland's Warbler in Michigan, cowbird control is now used in the management of several other sensitive songbirds, including the Least Bell's Vireo, Southwestern Willow Flycatcher, California Gnatcatcher, Black-capped Vireo, and Golden-cheeked Warbler.

The majority of research on the Brown-headed Cowbird has focused on its breeding behavior, interaction with host species, and habitat relationships. Research on general cowbird ecology or population-management strategies has been minimal. We know very little about the cowbird's seasonal movements, migration patterns, or population characteristics, particularly in southern California. While current cowbird trapping and removal projects have proven largely successful in the removal of target populations within limited areas on a seasonal basis, there has been no research into long-term trapping projects directed at regional cowbird population control through trapping year round or outside the breeding season. Considering the current rate of habitat destruction and fragmentation in California, cowbird parasitism will continue to be a central management issue with regard to the protection of sensitive songbirds.

In an effort to gain information on the cowbird's population characteristics and the feasibility of regional trapping and control programs, we began trapping and banding Brown-headed Cowbirds on a year-round basis at three locations within San Diego County in March 1997. Our primary objectives in this study are to determine if

  • Cowbirds resident during the breeding season focus on selected foraging areas before, during, or after the breeding season.
  • Cowbirds can be managed on a regional scale by trapping at selected foraging areas year round or during the nonbreeding season.
  • San Diego County cowbirds are permanent (year-round) residents or migratory.

Additional objectives include gathering information on the

  • Subspecies composition of San Diego County cowbirds
  • Seasonal movement and migration patterns
  • Site fidelity
  • General demography information (sex ratios, age classes, longevity).

Since March 1997 over 18,000 cowbirds have been captured in four traps located in the Tijuana River Valley, along the San Luis Rey River near Pala, and along the San Diego River near Lakeside. Approximately 6500 of these birds have been banded and released. Banded individuals have been recovered throughout San Diego County as well as in Orange, Riverside, Santa Barbara, Kern, Inyo, and even Lassen counties, California.

While the banding project is ongoing and a complete analysis of the project's data has not yet been initiated, preliminary information gathered thus far from the trapping and band recoveries indicate that

  • Cowbird numbers within San Diego County are at their highest during spring and fall migration, with peak numbers between April and mid May, and again from mid July through October.
  • Cowbirds resident during the breeding season are migratory and do not overwinter within San Diego County.
  • Cowbird numbers in the county are at their lowest during the winter, from December to mid March.

Both western subspecies of the Brown-headed Cowbird occur commonly in San Diego County, the Dwarf Cowbird (M. a. obscurus), as both a summer breeding resident and migrant, and the Great Basin Cowbird (M. a. artemisiae), as a common spring and fall migrant.

The sex ratio of captured cowbirds is 2:1, males over females, and juvenile cowbirds captured between July and December make up approximately 70% of the total captures.

The cowbirds' foraging areas (stables, dairies, livestock corrals) are used by summer resident cowbirds on a relatively regular basis, with individual banded birds traveling up to 15 miles between foraging locations and breeding areas.

In addition to providing unprecedented information on birds' distribution within San Diego County, the San Diego County Bird Atlas can also complement other research projects. While conducting atlas surveys, please note any banded cowbirds observed. Information recorded should include the date, location, sex of bird, and band color combination. If you see a banded cowbird, please include the information on your atlas data sheet and notify us at (760) 728-2152 or

We thank the USGS Western Ecological Research Center, U.S. Fish and Wildlife Service, California Department of Fish and Game, Cleveland National Forest, and California Department of Transportation (Caltrans) for their cooperation and support in this research project.

Thank you for your help.

--Jeff Wells and Jennifer Turnbull, TW Biological Services

The Last Holdouts

Via our bird atlas e-mail list-server, Ken Weaver recently posed the question, "what species of birds expected to nest in San Diego County still have not been confirmed doing so since we initiated the atlas in 1997?" The question sparked an exchange of interesting ideas and elicited from Bill Haas a nesting record of Cassin's Vireo on Middle Peak (M20) in 1998 that had almost slipped through our fingers. Frank Unmack followed through with the politicking getting him to an active nest of the Harris' Hawk in the McCain Valley area (S26)—more on this exciting and evolving story in the next issue. Carol and Dennis Wysong and Carol Manning noted a White-faced Ibis carrying nest material at the pond just southeast of the intersection of Highway 76 and Interstate 15—a new site for this species. (Dennis Parker of the county parks department is also arranging a boat so we can determine definitively whether ibises are still nesting at Guajome Lake). That reduces to 12 the number of species listed on the daily field form for the breeding season not confirmed nesting since 1997. As evidence of the overwhelming effort and accomplishments of our participants, these include no common or widespread species. Indeed, some of the species have never been confirmed nesting in the county; my including them on the list was a projection based on their occurring in the past through the breeding season.

The 12 "last holdouts" are these:

Northern Pintail. Never a regular nesting species, with just a few occasional records along the coast. Since we initiated the atlas, a few pintails have spent the summer at inland lakes (Morena, Cuyamaca, Sweetwater) as well as along the coast at Los Peñasquitos Lagoon without betraying any breeding behavior.

Zone-tailed Hawk. The one pair known to have nested in San Diego County, on Hot Springs Mountain (E20), has apparently not been reported since 1993. But the number of sightings between Escondido and San Pasqual on the south, Nate Harrison Grade, the San Luis Rey River, and Mesa Grande on the north, suggest that another nesting pair could be hidden somewhere in the rugged hills between, possibly on the private Rancho Guejito, to which we still have no access.

Ring-necked Pheasant. This introduced species may not be sustaining itself in the wild at most places where it is seen but only through continuing introductions. Kirsten Winter notes that pheasants are released regularly at private ranches in Pine Hills (K19) and along the San Luis Rey River (F16); she doubts they survive long enough to breed.

Spotted Dove. Yet another nonnative species, the Spotted Dove was once locally numerous in San Diego County. Now, for unknown reasons, it is experiencing a population collapse that has resulted in its range essentially retracting out of the county. Though it was fairly common 30 years ago in Encanto and Spring Valley, our only recent record for that area is of one singing bird I saw in Encanto (S11) on 21 June 1997. I have covered this area well since then so I must conclude that bird was the last. Our only other record during the breeding season is of two seen by Dave Seals near the Riverside County line in Rainbow (C9) on 19 April 1999 (Dave and Ken Weaver have also recorded the species there in winter). Now birders in Riverside and Orange County are commenting on the mysterious disappearance of the Spotted Dove there.

N. Pygmy Owl. Despite all our effort toward the atlas, the status of the Pygmy Owl in San Diego County remains enigmatic. Our only two records, both from Palomar Mountain, are by some of our most experienced observers (D15, 13 May 1999, Ken Weaver; E15, 28 May 1999, Ed Hall, Clark Mahrdt, and Jim Zimmer) but are still based on calls only. Given that the Pygmy Owl is active by day, if there were enough of a population to be self sustaining, I would expect at least a few visual contacts.

N. Saw-whet Owl. Though the Saw-whet Owl is not a rare bird in montane coniferous woodland, at least on Palomar Mountain, its small size and strictly nocturnal habits render it difficult to see, much less observe nesting. There is only one nesting record for the county even in the historic literature, and the egg collectors of the early 20th century evidently never found a nest. The closest we have come so far during the atlas period is Bill Haas' observation of one calling near a recently installed nest box (advertising it to possible mates?) near the San Luis Rey River at Wigham Creek (F16). Apparently it was unsuccessful and moved on. The Saw-whet Owl appears to be the commonest bird in San Diego County not yet confirmed nesting for the atlas.

N. Am. Dipper. The Dipper remains an enigma, to me, at least. We have no records for the atlas, and the last dipper seen in the county by birders may be the one Margaret and Bert McIntosh and I had in 1988 along the San Luis Rey River below Lake Henshaw (G16). Yet trout fisherman assert they still persist. The only way the species will be found is through an expedition focused on it, along Pauma Creek descending the steep slope of Palomar Mountain or possibly along the west fork of the San Luis Rey River between Barker Valley and Lake Henshaw.

Juvenile Swainson's Thrush
Illustration by Nicole Peretta

Swainson's Thrush. Please see the summer 1999 issue of Wrenderings for a review of this secretive species of dense riparian woodland, common in migration but rare as a breeding bird here at the southern edge of its breeding range in San Diego County.

Golden-crowned Kinglet. In seeming defiance of global warming, this bird of cold climates colonized San Diego County's highest mountains only recently, in the mid 1980s. Its nesting in the county has never been confirmed. Our only summer records since the initiation of the atlas have been from Cuyamaca Peak, though it has also occurred in summer on Hot Springs Mountain. Since there are only a few individuals of a species that lives and nests in the treetops, frequently detected by call only, it's not surprising that the Golden-crowned Kinglet is a holdout.

Fox Sparrow. In the breeding season the Fox Sparrow is also a mountaintop species, known from Cuyamaca Peak and Volcan Mountain. Dense undergrowth screens its nests from the prying eyes of birders as well as predators—the Fox Sparrow has never been confirmed nesting in the county either. But in July its fledglings should be conspicuous enough to make this species less of a challenge than the kinglet.

Yellow-rumped Warbler. Yet a third mountaintop breeder, also an apparent recent colonist on Cuyamaca Peak and Hot Springs Mountain. It may occur only irregularly, and the few scattered summering birds may be unable to find mates. Apparently there has been only one record of confirmed breeding in the county, Guy McCaskie's observation of a pair feeding recently fledged young on Hot Springs Mountain on 12 July 1986. Our few reports from possible breeding habitat since the initiation of the atlas are from Cuyamaca Peak only.

Wilson's Warbler. San Diego County was at the southern tip of the Wilson's Warbler's breeding range, and with the invasion of the Brown-headed Cowbird early in the 20th century that range retracted north out of the county. Since 1997, we have only two records of summering Wilson's Warblers, Paul Jorgensen's of a pair on Palomar Mountain (D14) on 28 July 1997 and Robbie Fischer's of one along the lower Santa Margarita River in Camp Pendleton on 19 June 1999.

I hope this list will inspire some of you to track down these difficult and elusive species!

--Philip Unitt

The Helicopter Trip to the Santa Rosa Mountains—Take 3

Thanks again to the efforts of California State Parks biologist Paul Jorgensen, we mounted another expedition to the Santa Rosa Mountains from 2 to 4 May 2000, in tandem with other of Anza-Borrego's tasks requiring a helicopter. Again, park ranger Bob Thériault went to square C28, Lori Hargrove went to D28, and I went to C27. In addition, Ginger Rebstock went to Alder Canyon in C21 and Jeff Wells went to Middle Willows in C22, as part of his monitoring of Bell's Vireos as well as gathering information for the atlas.

We learned that we must be absolutely sure of where we're going before the helicopter drops us off. The steep southwest face of the Santa Rosa Mountains looks very uniform as you approach it from the air, and as the pilot circled over the summit I had only a few seconds to decide if we were in the right spot. We weren't—I got dropped on Rabbit Peak in Riverside County instead of Villager Peak in San Diego County as intended, and though I realized the mistake within 5 minutes, by then the helicopter had continued on, taking Lori to her destination. Fortunately, Lori, Bob, and Jeff ended up in their right places, and if we had intended to send Ginger to Sheep Canyon in D22 instead of Alder Canyon in C21, well, we needed better coverage of C21 just as much.

Once I resigned myself to the helicopter's not returning to rectify the mistake, I started the hike from Rabbit Peak into San Diego County. The distance is barely more than a quarter mile and the hike takes only 20 minutes, but the drop is 600 feet down a steep rocky slope with virtually no trail. Once at the county line, however, at an elevation of 6000 feet, I found myself on the edge of the densest pinyon woodland in San Diego County. Apparently just one pair of Mountain Chickadees crosses the line, along with two territories of the California Thrasher. Interestingly, the Pygmy Nuthatch, plus Merriam's Chipmunk, occur on the summit of Rabbit Peak (elevation 6623 feet) but not farther south in San Diego County. Again, no Pinyon Jays.

Bob, at a lower elevation along the east base of the mountains, did the best of any of us, clearing the threshold for square C28 in this single three-day visit. Lori, at elevations of 4100 to 5260 feet, also confirmed several species' nesting, while I had only a few (Ash-throated Flycatcher, Bushtit, and Rock Wren, besides old nest holes of the Ladder-backed Woodpecker). Thus it appeared that early May was prime time for nesting at the low elevations but was still early at the highest elevations. A lot of spring migrants were moving through—I saw dozens of warblers each morning up to around 9 AM, all flying northwest, barely stopping in the pinyon trees before clearing the ridge and heading out over the abyss. Perhaps our most notable likely breeding species was the Violet-green Swallow. I had one pair circling a cliff face, and Lori had several, landing on snags with cavities and even on the ground. The most notable nonbreeding species was surely Lori's Lewis' Woodpecker, on 4 May not only late for this winter visitor but seemingly off any expected migration route.

-- Philip Unitt

Summer 2000 Wrenderings | Wrenderings Archive | Bird Atlas Introduction