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Warbling Vireo (Vireo gilvus)
Males of this rather drab vireo have a wonderful undulating song that they give throughout the day during much of the breeding season, often from the nest (Elliott 1901).  Two subspecies groups, western swainsoni and eastern gilvus, differ in size (especially the bill), mitochondrial DNA, and molt scheduling and probably should be treated as separate species.  Eastern birds complete their entire molt on the breeding grounds before migrating south, whereas western birds begin body molt on the breeding grounds and then migrate to Mexico to replace their flight feathers and the rest of their body feathers, the process taking about two weeks longer than that of eastern birds (Voelker and Rohwer 1998).  Warbling Vireos breed from s. Yukon, sw. Northwest Territories, and se. Alaska southeast to s. Ontario, s. Quebec, New Brunswick, and Nova Scotia and throughout much of the contiguous U.S. exclusive of the southeastern states; their breeding range extends south along the mountains of mainland Mexico to Guerrero and Oaxaca and to Baja California Sur.  They winter from Baja California Sur and w. Mexico south through the Isthmus of Tehuantepec to Guatemala, El Salvador, Honduras, and extreme nw. Nicaragua.

Subspecies: V. g. gilvus and V. g. swainsoni

Status and Occurrence: Fairly common to common breeding resident statewide, being especially numerous in the western half of the state.  The species usually arrives in mid- to late May and leaves from mid-Aug to early Sep.  The earliest sighting was by Tammy James at Glasgow on 6 May 2004 and the latest by Steve Hoffman at Helena on 29 Oct 2009 (the only Oct record for Montana).  Most nesting occurs from late May to late Jul.  Egg dates for 163 nests in the Bitterroot Valley from 1995 to 1997 ranged from 13 May to 18 Jul; some of the later nests were second clutches (J. Tewksbury, pers. comm.).

Warbling Vireos from the 19th century are well represented in museum collections.  The first ones were obtained along the Clark Fork R. in 1860 by C. B. R. Kennerly on 7 Jun (USNM 21946) and J. G. Cooper on 22 Aug (USNM 39361).  J. A. Allen collected one near Fort Sarpy on 9 Aug 1873 (MCZ 23796), and Elliott Coues took one near Waterton Lake on 19 Aug 1874 (USNM 67626).  R. S. Williams obtained one in the Little Belt Mts. on 26 Jun 1880 (MCZ 189138), six along Belt Cr. from 8 to 10 Jun 1889 (MCZ 189132-189137), one near Great Falls on 19 Aug 1889 (MCZ 189139), and one each near Columbia Falls on 11 and 13 Jun 1896 (MCZ 189140, 189141).  Other historical specimens were taken by C. W. Richmond in Gallatin Co. from 25 Aug to 10 Sep 1888 (USNM 123060-123062, 123064, 123065, 176691, 524806; RBCM 5275), Platte M. Thorne at Fort Keogh from 28 May to 24 Jul 1889 (USNM 172138-172141), and A. H. Howell at St. Mary Lake on 27 May 1895 (USNM 137832).

Habitat:  Warbling Vireos are associated with deciduous woodlands throughout their range.  In Montana, they occur commonly in aspen groves, riparian cottonwood forests, and regenerating clearcuts and less often in conifers with deciduous shrubs and roadside vegetation (Hutto and Young 1999).  During migration they also use urban parks and residential areas with large trees.  The nest is a hanging cup suspended from a fork in a horizontal limb, usually well away from the main trunk and from 1 to 30 m above ground (Gardali and Ballard 2000).  The mean height of 27 nests found in the Bitterroot Valley was 4.4 m (Banks and Martin 2001).  The species forages at all heights in trees and shrubs and gleans insects from foliage.

Conservation: Level III Priority in Montana owing to its vulnerability to cowbird parasitism.  That said, the Warbling Vireo generally is not of high conservation concern, and numbers have been stable in most of its range.  BBS data from Montana indicate a nonsignificant increase in numbers of 0.6% per year from 1966-2009. Numbers increased significantly by 0.8% per year survey-wide during the same period.  The global population estimate is 22 million birds (Rich et al. 2004).

In Montana and elsewhere, Warbling Vireo nests are heavily parasitized by Brown-headed Cowbirds (Tewksbury et al. 1998, Gardali and Ballard 2000, Banks and Martin 2001).  Some pairs in Montana reject cowbird eggs (Sealy et al. 2000), but productivity is reduced substantially at nests where a cowbird egg hatches (Tewksbury et al. 1998, Ward and Smith 2000).  The species would decline following wholesale removal of cottonwood and aspen stands, but it seems to benefit from the shrubby habitats that develop after coniferous forests are harvested (although productivity in such habitats is unknown; Hutto and Young 1999).

Historical Notes:  Little was written about Warbling Vireos in the 19th century.  Hayden said only that they were “Abundant along the wooded bottoms of the Missouri” (1862: 162), Cooper remarked that they were “Rather less common than the preceding [Red-eyed Vireo] in the Rocky Mountains” (1869b: 35), Allen noted that they were “Common…wherever there is timber” (1874: 54), and Coues observed that the specimen he obtained near present-day Glacier NP was “probably of the slight variety swainsoni” (1878: 575).  Richmond and Knowlton noted that they were “Common in the willows and cottonwoods along streams in the valleys” in Gallatin Co. (1894: 397).  In contrast, Thorne (1895) considered them uncommon breeders near Fort Keogh.

Silloway noted that Warbling Vireos were abundant near Bigfork in the summer of 1900 and found seven nests between 14 and 26 Jun.  All were placed 1.5 to 2.75 m above ground in clumps of deciduous trees.  Each nest contained three or four eggs except for one that had three young ready to fledge on 23 Jun.  He remarked that the birds continued to sing “when all other songs had been hushed by the mid-afternoon heat” (1901a: 50).  Cameron did not believe that Warbling Vireos nested along the lower Yellowstone R., stating that they were “Tolerably common in spring…but I have not found a nest nor observed this bird after the end of May” (1908a: 49).  He also made the unusual observation that they ate wild rose leaves.

Contemporary Work:  Kroodsma (1973) tallied 91 individuals in 23 riparian forest tracts along the lower Missouri and lower Yellowstone drainages in 1968 and 1971.  Warbling Vireos were slightly more common than Red-eyed Vireos but much less abundant than Yellow Warblers and Yellow-breasted Chats.  They were the second-most commonly detected species in riparian stands along the Madison R. near Ennis, where 134 pairs were found in a 24-km segment of the river in 2003 and 2004 (Marks et al. 2006).  They were less numerous along a 48-km stretch of the Tongue R. near Ashland in 2005, with only 36 breeding pairs found on 13 of 16 survey units (Marks and Brown 2005).

Alison Banks assessed how host activity near the nest affects cowbird parasitism for Dusky Flycatchers, Warbling Vireos, Yellow Warblers, and American Redstarts in the Bitterroot Valley in 1995 and 1996 (Banks 1997, Banks and Martin 2001).  Warbling Vireos were parasitized at the highest rate among the species, with 68% of nests receiving an average of 1.3 cowbird eggs.  The amount of activity near the nest was positively correlated with parasitism rates across host species.  Compared with the other species, female Warbling Vireos spent more time on and near their nests, especially during nest building, and males sang more near their nests.  Banks suggested that the high amount of activity by male and female vireos in the immediate nest area resulted in higher parasitism by cowbirds.

Also in the Bitterroot Valley, Josh Tewksbury assessed how forest fragmentation affects nest predation and cowbird parasitism in 20 passerine species from 1995-1997 (Tewksbury et al. 1998, 1999).  Owing to high cowbird parasitism in fragmented landscapes and high nest predation in forested areas, Warbling Vireos had low productivity, fledgling only one young per nesting attempt.  At that level of production, individual pairs would need to attempt three to four nests per year to maintain a stable population (Tewksbury et al. 1998).

Sponsored by Jim Rogers and Sherry Jones, Polson

Copyright Notice: © 2011. Jeff Marks, Dan Casey, Paul Hendricks.
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