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"Because he is clad in Quaker gray there is little need for the Vireo to show himself as he sings, and he remains for the most part concealed in the dense foliage, a vocal embodiment of the living green..." -Dawson

Identification
The Warbling Vireo is a 14-cm. drab gray to olive-gray bird with lemony-yellow flanks, white underparts, and blue legs. It is one of the white-eyebrowed vireos like the Philadelphia, Black-whiskered and Red-eyed Vireos found in eastern North America. At least in California, Warbling Vireos are the only vireos without wingbars. Males and females of this species look identical, and the male even develops a partial brood patch during the breeding season.

Subspecies
The subspecies of Warbling Vireo are Vireo gilvus gilvus of eastern North America, and the smaller V. g. swainsoni on the Pacific Coast, which is the one occurring in Andrew Molera State Park.

Range
The Warbling Vireo is a neotropical migrant, meaning that it spends its summer (breeding season) in North America, but migrates to Central and/or South America to spend the winter. Its summer range is almost throughout North America, excluding Nunavut, most of Alaska, Quebec and Ontario, and small regions of the lower 48 States.

Song
The western swainsoni vireos have a more melodious song than the eastern gilvus vireos. The song is a rapidly alternating (ascending and descending) series of sharp notes - a "warble". Warbling Vireos are probably more often heard than seen: though common in some areas, they may be difficult to see in the high treetops.

Banding History
As of February 8, 2001, the Big Sur Ornithology Lab had banded 1804 of 2404 Warbling Vireos captured in its mist-nets. The earliest date of capture was March 14 and the latest date was November 15, though that was probably an exceptionally late straggler. The last Warbling Vireos captured each year otherwise occur in October, the latest on October 18. The Big Sur Ornithology Lab will be analyzing its 9 years of banding data to study (if any) trends in Warbling Vireo productivity and survival over time in Andrew Molera State Park.

Habits
Warbling Vireos are common summer residents of deciduous and mixed forests; in Monterey County they favor forests of live oaks, California laurel, tan oak, and madrone trees, especially shaded canyons with riparian vegetation. Redwood canyons with live oaks and laurels support good populations in Big Sur. Higher montane coniferous forests support smaller populations with territories centered around a broadleaf tree. This vireo also inhabits riparian forests of willow, cottonwood, and sycamore. Warbling vireos are insectivores.

The nest is usually high up (20-60 feet) but here in the west can be found in shrubs as low as 4 feet. Like in other vireos, its nest is a pensile cup bound to a skirt of twigs around the rim of the nest. The nest is made of hair, long grasses, threads and string, bark strips, plant down and lichens. It is bound with spider webs and lined inside with fine shredded weed stems.

Both sexes share the task of incubating the eggs and feeding the nestlings. Breeding biology of this species in Monterey County is poorly known. According to Baicich and Harrison (1997), breeding begins mid-May and birds are double-brooded, usually 4 eggs per brood; however, in Monterey County, vireos have been seen being accompanied by begging fledglings as early as May 12, suggesting that breeding can begin in April. Incubation is 12-13 days. Eyes open 9 days after hatching. The length of the nestling period remains uncertain but may be around two weeks. Data from the Monterey Breeding Bird Atlas suggests a total nesting cycle length of 40 days.

Status
The Warbling Vireo isn't a threatened or endangered species. In some parts of North America it is increasing while declining elsewhere on the continent. Declines have been observed here in Monterey County, and are attributed to the loss of riparian forests where it breeds, and to increased brood parasitism by Brown-headed Cowbirds. The decline in many songbird species, including other vireos like Bell's Vireo (V. belli) and the Black-capped Vireo (V. atricapillus) has been linked to loss of breeding habitats and increases in brood parasitism.

A coalition of public and private organizations called Partners in Flight (PIF) is trying to reverse the population declines of many North American Songbirds. The California chapter of PIF has produced a document called the California Riparian Habitat Conservation Plan, which outlines suggestions for restoring California's riparian bird species by preserving and actively restoring riparian habitats and reducing the impacts of human industry and brood parasitism. The Plan is completely voluntary.

The Warbling Vireo is a focal species in this Plan, that being one of a few selected indicator species whose populations may be intensively monitored to assess the health of a particular ecosystem. Economy of effort is the rationale behind monitoring a few carefully selected focal species: if the ecosystem is largely healthy, the populations of many more unmonitored species are probably healthy as well.

- Lionel Leston

References

VWS Featured Birds:
Townsend's Warbler
Yellow Warbler
Flycatcher
Warbling Vireo
Wilson's Warbler