by John Shackford
According to USGS Breeding Bird Survey data from 1966-2012, and personal observations, the Bell’s Vireo (Vireo bellii) has shown a drastic decline since the 1960s: the primary problem seems to be Brown-headed Cowbird brood parasitism.
When I moved from North Carolina to Oklahoma City in 1958 I was eager to find species that were new to me. I do not remember when I first saw a Bell’s Vireo, but I am confident that by the mid-1960s I had a pretty good bead on them, and they were quite common, not too hard to find at all. About this time Dr. George Sutton (1967, Oklahoma Birds) gave the range in Oklahoma of the Bell’s vireo as “locally abundant in central Oklahoma, rare in Panhandle and in heavily wooded parts of eastern Oklahoma…,” a summary of the population as of 1967; this also shows the Bell’s Vireo to be a special bird for central Oklahoma. Sutton goes on to say they “inhabit plum thickets, bois d’arc and mulberry shelter belts…and stands of salt cedar and of sapling elms, persimmons, willows, and locusts”.
The main reason I am connecting this species to a fall Bird of the Month report has to do with banding the bird at Lake Overholser in the fall perhaps 25 years or so ago (Yikes, has it been that long ago?). Patti and Brian Muzny, Nancy Vicars, John Newell, Warren Harden (who had the banding license), Wes Isaacs, Hubert Harris, Ted Goulden and several others, including me, would often band on fall Saturday mornings during this period; we did this over a number of years. We cut extensive mist net paths—maybe 8-10 netsworth—in the small willows on the north side of the coffer dam road at the north end of the lake. In our banding area we would hear quite a few vireos, apparently congregating somewhat in the fall prior to migration. We would band several toward the end of summer, but then—overnight—the Bell’s Vireos would—as a group—disappear. We might be able to find a single bird or so after this disappearance date, but almost the entire flock had launched off the same night, apparently, toward the wintering grounds along the west coast of Mexico.
Vickie Byre, who was a remarkable ornithologist from Norman before her untimely death a number of years ago, found Bell’s Vireos fairly commonly along the South Canadian River when she was studying nesting Least Terns there during several years in the 1990s. The vireos were nesting in the salt cedar thickets along the edge of the river. But coming up to the present, the Bell’s Vireo has become decidedly rare.
To the best of my knowledge, there just are not many places where you can depend on finding nesting Bell’s Vireos anymore in central Oklahoma. So it was a pleasant surprise that on a field trip to Mitch Park last spring with Hal Yocum several other people with very sharp ears heard 2-3 Bell’s Vireos during a hike in the park, but my hearing is now such that I missed them.
The song is given by Kenn Kaufman (2000, Field Guide to Birds of North America) as ”fast jumbled series, cheedledoo-cheedledeele-dee?…cheedledoo-cheedledeedle-doo!” From my recollection this is not a bad representation of what the song of this bird sounds like. The species lays 3-5 eggs, normally 4, in low brushy areas, usually nesting about 2-3 feet from the ground, but I found 1 reference to a nest that was about 8 feet above the ground.
The main problem for the Bell’s Vireo in the last 50 years–as mentioned above—has no doubt been brood parasitism by the Brown-headed Cowbird, abetted by overgrazing of pastures by cattle: the cattle tend to destroy low tree shrubs and this is just where the Bell’s Vireo nests.
The Bell’s Vireo was named by Audubon in 1844 for John Graham Bell, who accompanied Audubon on Audubon’s trip up the Missouri River in the 1840s. The Least Bell’s Vireo (V.b. pusillus), a subspecies, is found nesting in southern California and down into Mexico. It is an endangered subspecies, and cowbirds are the main problem for that subspecies too.