Who sang what now?

Bill Chaisson
Of a Feather
Last week I was stacking the last of the firewood when I heard the cry of a broad-winged. This is an odd call to hear in the second week of November. When I looked up, I saw a blue jay, which conveniently did it again as I watched. In the moment I recalled hearing a jay (perhaps the same one) make this call early in the spring. On that occasion I was doubly surprised because it was too early for the hawks to have returned and blue jays had been all but absent for the entire winter.

In North America vocal mimicry is associated with the corvids and the mimic thrushes, as well as the introduced sturnids. In Australia the lyrebirds and bowerbirds are vocal mimics; 53 out of 360 Australian passerine species are mimics. In Great Britain, 30 out of 160 passerines are mimics. In South Africa, 63 out 370 are mimics. One of the few characteristics these mimic species share is they are rarely migratory.

Why then do vocal mimics do what they do? In a 1982 volume, Acoustic Communication in Birds vol. 2, Jeffrey R. Baylis examined the evolution of mimicry. He noted that in 1896 the fact that birds imitate was cited as evidence that songs were learned rather than innate, but for the ensuing 40 years there was no discussion of its mimicry’s function.

A 1983 study of European starlings (Sturnus vulgaris) by Meredith West and A. Neil Stroud demonstrated that mimicry is learned and has a social component. Seven starlings were reared in three different ways. Two had regular interaction (IC) with their captors and were often spoken to and whistled at. Two birds lived in a cage in a household where they could see and hear their captors but were not spoken to directly (LC). Finally, three birds lived on a screened porch where they could hear but not see their captors (AC). LC and AC birds lived with a brown-headed cowbird.

Only the IC birds mimicked human speech, while the LC and AC birds did not, but they did mimic their cowbird companion’s calls. Although all the starlings were exposed to tape-recorded speech and whistles, none ever mimicked those sounds, only speech and sounds made by humans they interacted with directly. (Although one of the IC birds did learn to imitate the hiss of the tape machine.) West and Stroud concluded there was a strong social component to learning to mimic and perhaps, among starlings, it has a role in their social structure.

The three mimid thrushes range into the northeast U.S. — the northern mockingbird (Mimus polyglottos), brown thrasher (Toxostoma rufum), and gray catbird (Dumetella carolinensis) — all defend individual breeding territories and are not particularly social during the rest of the year. Because the mockingbird repeats each phrase 6-8 times, the thrasher only twice, and the catbird not at all, it is understandably easier to pick out the sounds of other birds (and non-birds) in the song of M. polyglottos. The mockingbird, true to its name, is the most ardent imitator. According to birdsoftheworld.org, the brown thrasher is supposedly less talented: It “only occasionally mimics other species; these imitations [are] generally crude. Songs or calls identified in Brown Thrasher songs include those of Chuck-will’s-widow (Caprimulgus carolinensis), northern flicker (Colaptes auratus), white-eyed vireo (Vireo griseus), tufted titmouse (Baeolophus bicolor), wood thrush (Hylocichla mustelina), and northern cardinal (Cardinalis cardinalis).” In the 19th century “it [was] the opinion of many ornithologists that the song [of the brown thrasher] . . . is richer, fuller, and definitely more melodious than that of polyglottos.”

Corvids are not singers per se. So, while they tend to imitate the calls of other birds rather than songs, they also mimic everything under the sun. Emily Davis, a blogger for Audubon South Carolina, has a captive bird she calls John Jay. In addition to mimicking Carolina wrens, crows, and red-shouldered hawks, John Jay’s repertoire includes the sound of a microwave, the chime that urges you to fasten your seatbelt, a smoke alarm, and the swoosh that a phone makes when you send a text message. He can also produce the screams of children playing and two kinds of dog bark. Davis notes that corvids are smart and in the wild generally deploy their imitative skills for a purpose, like warning other birds of danger by mimicking the call of a hawk.

In his survey of vocal mimics, Baylis could find very little in common among mimic species. He groped for generalizations. Most tend to have large song repertoires and to sing continuously. Many are polygynists (have multiple female partners). Almost all are passerines.

His 1982 paper listed a number of hypotheses for the adaptive function of mimicry, but none were satisfactory. The scientific method required Baylis to list chance as a possible reason for mimicry. That is, there is no adaptive purpose. Others proposed “shared environmental factors” as a reason two species would produce the same sounds. Little evidence has been found for this.

Because singing is a learned behavior, some have proposed individuals learned the songs of other species because they were somehow deprived of hearing the songs of their own species. For example, by being born late in the breeding season or being raised by foster parents of another species. Again, there is limited evidence for this.

Many vocal mimics are drab birds that attract mates through the complexity and continuous nature of their singing. Some studies have attempted to show that to get a mate mimics simply take an “everything and the kitchen sink” approach by adding the sounds of other species to their repertoire.

Baylis’s 40-year-old survey concluded in frustration: people just weren’t studying mimicry systematically! When I looked up references in birdsoftheworld.org species profiles of mimic species, I found very few recent studies. Electronic analysis of sound has improved, so a lyrebird study could establish how well they mimicked but it did not ask why.