Until 1997, the Solitary Vireo in the United States consisted of four subspecies. Since then, based on additional research, the Solitary Vireo has been split into three full species. The eastern and northern races were merged as the Blue-headed Vireo, the Pacific coast birds became the Cassin’s Vireo, and the Rocky Mountain form the Plumbeous Vireo.
All three species have been documented in Inyo County. Only the Plumbeous Vireo is known as a breeder and regular summer resident in the pinyon-juniper habitats throughout the county. The Pacific coast Cassin’s Vireo is a spring and fall migrant through the county and is more common in spring than in fall. The Blue-headed Vireo is a vagrant to our area with but two records, both from Death Valley National Park. One was at Panamint Springs 3 Oct 1989 (Jon Dunn, AB 44:164) and the other at Furnace Creek Ranch 23 September 1991 (Jon Dunn, AB 46:151).
All field guides make identification seem rather straightforward in separating these three taxa, and often it is. However, there are pitfalls that are underappreciated and lead to misidentifications. Vireos are brightest in early fall immediately after molt when their new feathers are unworn and colors are not faded by UV rays and wear. They are most drab and dull in summer just before fall molt. Many are fairly dull in spring with females always tending to less color than males. Thus, a young female in spring or summer when one year old may be particularly dull.
The biggest challenge for Inyo birders is trying to separate a dull Cassin’s from Plumbeous. We regularly receive reports of Plumbeous Vireo in April and even late March that undoubtedly are very dull, probably young female, Cassin’s Vireos. Cassin’s have been recorded as migrants throughout the state during April and a few earlier in late March with peak numbers in May. Plumbeous move later. The misconception is that if there is no yellow on the flanks then it must be a Plumbeous…or Gray, which is another article.
Think about the habitat in which Plumbeous breed, pinyon-juniper, and then what it is like in April at 7000-9000′. It is still winter-like and these conditions may prevail well into May preventing the insect activity that is necessary for the vireos’ food supply. Plumbeous Vireos winter, for the most part, in Mexico, although a few are known to winter along coastal California. The recently published Birds of the Salton Sea (Patten et al. 2003) lists two spring migrants sightings from late April and mid May as tentative only and state that “there are virtually no physically documented records at that season.” Considering the number of highly qualified birders working this popular birding destination, if Plumbeous were coming through in March and April there would be unequivocal records.
Any suspected Plumbeous Vireos in Inyo County prior to mid May should be meticulously documented with particular attention given to head and back color and contrast, flank color, and the color of the edges of the secondaries and tertials.
As if that isn’t enough, another problem involves separating very bright and freshly molted fall Cassin’s from Blue-headed Vireo! The Blue-headed has strongly contrasting gray to blue-gray head and green back, pure white throat cut off sharply with gray cheeks, and white in the tail. With only two accepted records great detail is required and as this is a reviewable bird by the California Bird Records Committee one must submit the documentation to the Committee for peer review.
Competent observers have reported Cassin’s Vireos that they thought were breeding in the Sierra based on singing birds that seemed to be territorial. Subsequent searches revealed that they were just late migrants. Both sexes of vireos are known to sing and reportedly throughout the year. Two excellent sources for detailed information and photographs on the identification of these three species are: 1) Heindel, M.T. 1996. Birding 28: p. 459 and 2) Zimmer, K. 2000. Birding the American West, p. 242.
The complexity and difficulty in separating some individuals in the Solitary Vireo complex rivals the infamous Empidonax flycatchers challenge. When in doubt claim a “Solitary Vireo” and you will ensure accuracy …unless it is a Gray Vireo!
Tags: flycatcher, vireo