(Originally published in the Mar-Apr 1998 issue of the Albatross, Vol. 43, no. 4)

 A Case of Mistaken Identity? 

By Jeff Davis

 On a late December visit to Anza-Borrego State Park in San Diego County, I encountered a Hutton's Vireo along the Borrego Palm Canyon Nature Trail. The vireo was in uncharacteristic habitat (creosote bush scrub) and out of range, though perhaps only by 25 miles or so. Even stranger to me, was that this vireo was being pursued by an agonistic male Ruby-crowned Kinglet. The kinglet employed the same aggressive displays it uses to chase off other male kinglets. As it perched close to the vireo, it flicked it wings, gave chattering calls, and leaned its body forward, with its head upright, red crown erect, tail slightly cocked, and wings fluttering. It pursued the vireo in this manner for more than 5 minutes while I watched. The kinglet was possibly just defending a winter feeding territory from a species that shares a similar foraging niche. It's interesting, though, that the two also are very similar in appearance. So similar, in fact, that without a careful look, a birder may easily mistake one for the other. According to the Ruby-crowned Kinglet monograph (No.119) published as a part of the Birds of North America series, interspecific territoriality (i.e., defense of a territory from an another species) is "not known to occur." If interspecific territoriality doesn't really occur, could it be that the kinglet made the same mistake a careless birder might?

 In a taxonomic sense, the kinglet and the vireo are not especially closely related, so it's even more interesting that the two look so similar. This sort of convergent evolution, as it's called, is not that uncommon among vertebrate animals occurring on different continents. For example, meadowlarks (related to blackbirds and orioles) in the grasslands of western North America look very similar to long claws (related to pipits and wagtails) in the grasslands of Africa. Likewise, convergence is not uncommon with respect to certain physical traits of birds occurring on the same continent. For example, the unrelated woodpeckers and owls have the same toe arrangement, with two toes facing forward and two backward. (Most birds have three toes forward and one backward.) However, it is rare for two unrelated species on the same continent to be so similar in overall appearance. The subjects of most, if not all, identification problems in North America are closely related, and are often even in the same genus (e.g., Aechmophorus grebes, Accipiters, Dowitchers, Empidonax and Myiarchus flycatchers, Dendroica warblers, Spizella sparrows, and Carpodacus finches). So, why do Hutton's Vireo and Ruby-crowned Kinglet, species not even in the same family, look so similar? Convergence, in general, is usually explained in terms of adaptations. That is, unrelated species that occupy similar ecological niches may come to look alike because it offers some advantage with respect to survival. These physical features are adaptive in that particular habitat and somehow render a higher degree of survival for individuals with that "look." Perhaps with their olive green plumages, similar to the colors of evergreen leaves in which they spend their time, they are less visible to predators. Like other convergent species, the kinglet and vireo do occupy similar habitats (evergreen
forests and woodlands) and eat similar foods (insects and spiders). However, they share these traits with many other species such as the Townsend's Warbler, which looks nothing like them. Clearly, that "look" is not the only one that works. Most field guides do an adequate job of describing ways to distinguish the two species, but sometimes give misleading information. Most notably, many sources cite a difference in behavior between the two. Although it may be true in general that the Ruby-crowned Kinglet is more active and flits it wings more often, the Hutton's Vireo can and does match the kinglet's level of activity . Below is a summary of the distinguishing characteristics.