A CLOSER LOOK—Mountain Chickadee

By Kimball Garrett

Originally published in the Western Tanager Vol. 46 No. 9 June 1980

Closer-Look-Mountain-Chickadee-illustration-by-Kimball-Garrett-scan

Previous "Closer Look" articles have treated southern California bird species which vary in their appearance according to age, sex, season, geographical locality, etc. But many other species, including this month's subject, the Mountain Chickadee, are remarkably uniform in their appearance. Subtle differences in measurements and in plumage pigment saturation in the Mountain Chickadee would surely be overlooked by even the keenest field observers. But the chickadee represents another type of within-species variation which is familiar to most bird-watchers—that is, geographical variation in vocalizations.

Bird vocalizations, particularly the more complex ones we call "songs," are subject to the same geographical influences that give rise to human dialects and accents. Bird species differ greatly in the degree to which their vocalizations vary between populations. In most shorebirds, for example, calls are quite uniform throughout a species' range. Dialects are quite common and marked in many passerines. One need only listen to the various recordings of Rufous-sided Towhees, White-crowned Sparrow, and other species on the Peterson Field Guide to Western Bird Songs to appreciate the degree to which songs of the same species may differ. Intensive studies on song variation and dialects have been made on the White-crowned Sparrow, the Cardinal, and other species with relatively simple songs. Analyses of very complex songs (e.g. House Finch, Fox Sparrow) have added much to our knowledge of the nature of song variation, but can be somewhat overwhelming when dozens or even hundreds of themes, notes, phrases, or syllables are involved. I have chosen to discuss the Mountain Chickadee here because its song is very simple (two to seven pure notes) and still shows prominent variation even within southern California.

Mountain Chickadees breed commonly in a wide variety of conifer-dominated forests and woodlands throughout the highlands of southern California (and elsewhere in the west). Isolated populations even occur on the desert ranges of the eastern Mohave. The basic territorial song of the male consists of from two to several clear whistles on two or more pitches. A crude sonagraph (plot of frequency versus time) might show a typical song as follows:

Closer-Look-Mountain-Chickadee-fig1

FIGURE 1

There are innumerable variations in this simple structure, but, as a general rule, neighboring birds tend to sing similar songs and therefore the observed variations can be thought of as dialects. I have listened to or tape-recorded Mountain Chickadee songs in several parts of the San Gabriel and San Bernardino Mtns., finding consistent dialects through the years. A small pocket of birds along the south shore of Big Bear Lake, in the vicinity of Cedar and Bluff Lakes, is always heard to give three descending whistles which perfectly follow the tune of "Three Blind Mice." This "dialect" seems to occur nowhere else in the San Bernardinos, despite the lack of obvious habitat or topographical boundaries which would prevent "cultural exchange" of songs. All singing males in this area give the "Three Blind Mice" song and that song only. The crude sonagraphs below depict this and other typical songs heard in the mountains of southern California:

Closer-Look-Mountain-Chickadee-fig2

FIGURE 2 (crude sonagraph)

Observers who spend time within the breeding range of the Mountain Chickadee may find it rewarding to listen to their songs (singing peaks from April through June) and record or graphically represent them. The answers to a number of questions can be sought. Do all birds in a given area (population) give the exact same song? What is the degree of individual and intra-population song variation? Are boundaries between "dialect" types sharp? If dialect boundaries are sharp, do they follow any obvious barriers, bridges, valleys, tracts of unsuitable habitat)? Does the dominant dialect in a given area change through time? Crucial to an understanding of the ecological and evolutionary significance of song dialects in the Mountain Chickadee and any other species is the determination of how the song is acquired. Is the song innate, such that birds of a particular genetic make-up will sing a particular song (this rarely seems to be the complete case in passerines)? Or, conversely, is the song entirely learned from other members of the same species in the area? Most passerines fall at an intermediate stage of this continuum. Their songs have both innate and culturally-transmitted components. At what stage might song-learning take place in the Mountain Chickadee? Perhaps some exceptionally enthusiastic reader may even wish to try to whistle his or her favorite theme into the repertoire of a Mountain Chickadee population. This has often been done in captive birds at a sensitive young age.

Evolutionary explanations for the existence of song dialects in birds are varied. Some scientists feel that local song variations are adapted to fit the physical landscape (e.g. certain frequencies travel better at certain altitudes or through different vegetation types); while this is undoubtedly true most researchers prefer to stress the cultural and ecological bases for dialects. Consider, for example, that a particular population of birds is likely to be genetically most suited to living in its own immediate physical and biotic environment. Other populations fare best in their subtly different environments. Song dialects could then act as a sorting mechanism by which individuals may recognize members of their own population and restrict outbreeding with individuals which are less well adapted to the immediate environment. Such a theory may be intellectually satisfying, but is very difficult to test. Whatever the reasons, dialects are marked in the simple song of the Mountain Chickadee, and are quite easily studied. Readers are urged to spend some time on their next excursion into the mountains taking a closer listen at these common but interesting birds.