{"id":2778,"date":"2011-03-31T21:48:45","date_gmt":"2011-04-01T03:48:45","guid":{"rendered":"http:\/\/earbirding.com\/blog\/?p=2778"},"modified":"2011-04-02T20:09:39","modified_gmt":"2011-04-03T02:09:39","slug":"least-bells-vireo","status":"publish","type":"post","link":"https:\/\/earbirding.com\/blog\/archives\/2778","title":{"rendered":"“Least” Bell’s Vireo"},"content":{"rendered":"
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Bell's Vireo (subspecies arizonae), 3\/28\/2009. Photo by Dominic Sherony (Creative Commons 2.0).<\/figcaption><\/figure>\n

I’ve always been mystified by the breeding range of Bell’s Vireo.\u00a0 It inhabits three seemingly very different places — the dense deciduous tangles of the Great Plains, the mesquite thornscrub of southeastern Arizona, and the riparian chaparral of southern coastal California.\u00a0 Sure, they’ve all got lots of dense thickets, but so do lots of places in between.<\/p>\n

As far as I can determine, the only other birds that occupy Bell’s Vireo’s entire range are continent-crawling generalists like House Finch and Mourning Dove that can be found practically everywhere<\/em>.\u00a0 How can Bell’s Vireo be so forgiving of the differences between, say, a streamside plum thicket in South Dakota and a tangle of mesquite in the Arizona desert, without being able to tolerate the deciduous scrub of the Colorado foothills?<\/p>\n

Part of the answer, of course, is that each of these habitats boasts a unique population of Bell’s Vireo with unique habitat preferences.\u00a0 Great Plains birds are the yellowish, greenish nominate subspecies; Arizona is home to the much less colorful subspecies arizonae<\/em>; and coastal California hosts the endangered “Least” Bell’s Vireo,V. b. pusillus<\/em>, the grayest of them all.<\/p>\n

Recently, Elisabeth Ammon of the Great Basin Bird Observatory<\/a> asked me for help in determining whether there are any vocal differences between pusillus <\/em>and arizonae<\/em>.\u00a0 During the upcoming breeding season, GBBO will be surveying sites in Death Valley National Park for Bell’s Vireo, including an area where the species was found last summer.\u00a0 If it is found to breed in Death Valley, the national park’s management plan may depend on whether the birds are determined to be of the federally endangered “Least” subspecies or the commoner arizonae<\/em>.<\/p>\n

The Sibley Guide to Birds says no vocal differences are known between the subspecies, but the recently revised Birds of North America account<\/a> suggests otherwise:<\/p>\n

Geographic Variation<\/strong><\/p>\n

Little known. A comparison of the samples taken from California and Arizona show slight differences in repertoire size, song length and number of notes per song…. Field researchers subjectively report qualitative differences in songs in different regions.<\/p><\/blockquote>\n

Unfortunately, BNA’s claims of vocal differences are backed up by very little quantitative information — only the observation that “Least” Bell’s Vireo has an average repertoire size of 9.2 songtypes per bird, whereas arizonae <\/em>averages 10.6 songtypes.\u00a0 That’s a small difference indeed, one that means the repertoire sizes must necessarily overlap, and one that could even fall entirely within the margin of error.\u00a0 Even if further investigation confirms this average difference, it would be of zero use in field identification.\u00a0 And BNA says nothing further about differences in song length and number of notes per song.\u00a0 Thus, we’re left to investigate the most slippery category:<\/p>\n

Qualitative differences<\/h4>\n

The initial prognosis for this blog post was grim, since the \u201cBig Three\u201d internet repositories of bird sounds \u2013 the Macaulay Library<\/a>, the Borror Lab<\/a>, and Xeno-Canto<\/a> \u2013 contain precious few recordings of \u201cLeast\u201d Bell\u2019s Vireo. As of this writing, Macaulay and Borror lack them completely, and XC has only a few, all from Baja California Norte:<\/p>\n

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