Myron Charles Baker

The Biology of Bird-Song Dialects

No single theory so far proposed gives a wholly satisfactory account of the origin and maintenance of bird-song dialects. This failure is the consequence of a weak comparative literature that precludes careful comparisons among species or studies, and of the complexity of the issues involved. Complexity arises because dialects seem to bear upon a wide range of features in the life history of bird species. We give an account of the principal issues in bird-song dialects: evolution of vocal learning, experimental findings on song ontogeny, dialect descriptions, female and male reactions to differences in dialect, and population genetics and dispersal. We present a synthetic theory of the origin and maintenance of song dialects, one that accommodates most of the different systems reported in the literature. The few data available suggest that large, regional dialect populations are genetically differentiated; this pattern is correlated with reduced dispersal between dialects, assortative mating by females, and male-male exclusion. At the same time, “subdialects” may be formed within regional dialects. Subdialect clusters are usually small and may represent vocal mimicry among a few adjacent territorial males. The relative importance of genetic and social adaptation may contribute to the emergence of subdialects; their distinctiveness may be correlated with the degree of polygyny, for example. Thus, subdialect formation is linked to one theory of the evolution of repertoire size, but data are too fragmentary to examine this idea critically.

SONG DIALECTS AND GENETIC DIFFERENCES IN WHITE-CROWNED SPARROWS (ZONOTRICHIA LEUCOPHRYS)

Song dialects have been described in several bird species (Marler and Tamura, 1962; Lemon, 1966; Nottebohm, 1969). Clear examples are found in the resident white-crowned sparrows (Zonotrichia leucophrys nuttalli) of coastal California and in populations of the chingolo (Zonotrichia capensis) of Argentina. Where dialects occur in these species an area of breeding habitat is occupied by territorial males that with few exceptions sing a very similar song. Contiguous and allopatric with this area males of another population sing a different song type. It has been shown (Marler and Tamura, 1964; Marler, 1970) that in the white-crown (Z. 1. nuttalli) the dialect is learned during the first few weeks of life. The contiguity of the populations with discretely different vocalizations could be related to several interesting questions in population genetics, vocal communication, and evolutionary processes. On a general level, however, the dialect phenomenon leads to two central issues: (1) How do dialects arise? and (2) What role, if any, do they play in the biology of the birds? In this paper I provide a speculation on the first question and some data that may clarify the second. Following a suggestion by Marler and Tamura (1962), Nottebohm (1969) proposed that females might preferentially mate with males of their own dialect, thereby constraining a locally adapted gene pool. This hypothesis, that dialects reduce gene migration, came from the observation that Z. capensis on altitudinal gradients in Argentina exhibits a series of dialect populations possibly corresponding to different selective regimes, whereas a single dialect is found over vast areas of lowland pampas where selection may be spatially homogeneous (Nottebohm, 1969; Nottebohm and Selander, 1972). There, are other possible explanations of song dialects in birds. First, song dialects, a consequence of vocal learning, may play no important role. Perhaps during the early learning phase, young birds acquire population specific calls that promote flocking, foraging efficiency, predator avoidance, or individual recognition, that are important for winter survival. Thus, the selective advantage of vocal learning would be explained by winter ecological conditions and not territorial breeding behavior. Second, dialects could maximize outcrossing in small populations. Females would mate with males having a song type unlike that of their early experience. Third, dialects may be a result of intrasexual selection and bear no direct relationship to mate selection. Perhaps vocal convergence bestows some advantage in gaining and holding a territory but females do not use dialect information for mate choice. Using an electrophoretic analysis of alloenzymes, I investigated populations of Nuttalls white-crowned sparrow (Z. 1. nuttalli) in California and populations of the mountain white-crown (Z. 1. oriantha) in Colorado to determine if song dialects were correlated with genetic differentiation.

Early Experience Determines Song Dialect Responsiveness of Female Sparrows

In a laboratory experiment, female white-crowned sparrows responded almost exclusively to male songs taken from their home dialect region and usually not to songs taken from an alien dialect region. Song dialect populations may represent a level of genetic population structure below that of the subspecies and may play an important role in songbird evolution.

Foraging success in junco flocks and the effects of social hierarchy

The relative advantages of flocking for socially dominant and subordinate dark-eyed juncos (Junco hyemalis) were investigated in a laboratory experiment. Success in finding seeds hidden in an artificial tree was determined for all individuals hunting alone. When hunting alone, birds who spread their searching more evenly over the habitat had a greater chance of finding the food source. Flocks of four were housed together and subsequently each flock formed a dominance hierarchy social structure. High-ranking individuals obtained more seeds when in flocks than when hunting as isolates. In numbers of seeds obtained, socially low-ranking birds gained no advantage in comparison to their performance as isolates. In our experiment, the primary advantage of flocking for low-ranking individuals was the reduced chance of obtaining no food at all.

Niche Relationships Among Six Species of Shorebirds on Their Wintering and Breeding Ranges

The dynamics of the organization of a community of six species of migrant predatory shorebirds (Least Sandpiper, Semipalmated Sandpiper, Dunlin, Short—billed Dowitcher, Lesser Yellowlegs, and Semipalmated Plover) was appraised by studying foraging behavior and habitat utilization under winter conditions in southern Florida and under summer conditions in the eastern Canadian Arctic. Eight foraging methods, defined primarily on the basis of how the bill is used and the pattern of locomotion, constitute the behavioral repertoire of the species. Each foraging method is correlated with a particular rate of locomotion (distance/time) and rate of feeding (pecking or probing/time). Feeding and locomotion rates seem independent of air temperature, number of conspecifics, and total number of shorebirds foraging nearby. Instead, the seasonal changes in these rates are probably related to food density. On the basis of these findings and of differences in rates of feeding and locomotion between species the most reason...

SONG DIALECTS AS BARRIERS TO DISPERSAL IN WHITE-CROWNED SPARROWS, ZONOTRICHIA LEUCOPHRYS NUTTALLI

Studies of avian communication systems have revealed an impressive number of cases in which a bird species exhibits a mosaic pattern of clusters of males singing a song or songs peculiar to each cluster (the most intensive study is that of Baptista, 1975). In some species, these subgroups are discrete, but in other species the differences between subgroups gradually grade from one to another. The geographic dimensions of these so-called dialects may be quite local, a few km 2 (Marler and Tamura, 1962), or more regional, a few hundred km 2 (Nottebohm, 1969). No unifying theory exists to explain the diversity of dialect systems, but several workers have suggested that mating probabilities might be influenced by song dialects with the effect of reduced genetic exchange among dialect groups (Nottebohn, 1969; Baptista, 1975). In the few species carefully studied, it has been found that the song dialect is learned early in ontogeny (Marler and Tamura, 1964; Mundinger, 1975). The dialect phenomenon suggests two kinds of questions. First, how do song dialects originate? One view of the historical origin of a dialect system is analogous to a sympatric speciation model in which dialects arise within a continuously distributed population with the passage of time (Nottebohm, 1969). An alternative proposal, essentially an allopatric model, is that dialects arise from a colonization or

The behavioral response of female Nuttall's White-crowned Sparrows to male song of natal and alien dialects

SummaryFemale Nuttalls White-crowned Sparrows (Zonotrichia leucophrys nuttalli) were tested in a laboratory experiment to determine their response to stimulation by songs from their natal dialect (Clear) and by songs from three different alien dialects (Fig. 1). The greatest number of copulation solicitation displays and amount of locomotor activity were caused by songs of the subjects natal dialect (Clear); the next most potent stimulus was the contiguously adjacent dialect (Buzzy); the least effective stimuli were songs from a dialect 25 km distant (McClure), representing the same non-migratory subspecies, and a dialect 1,900 km distant (Sand Creek), representing a migratory montane subspecies of White-crowned Sparrow (Figs. 2–4).These results constitute a test of a deduction made from the assortative mating theory and suggest that female White-crowned Sparrows find male song of their natal dialect sexually more stimulating. Thus, speculating about the natural circumstances, females given an unrestricted choice would be expected to mate with males from their natal dialect region.

Dominance, survival, and enzyme polymorphism in dark-eyed juncos, Junco hyemalis

Although birds have played a major role in the construction of ecological and evolutionary theory, the genetic structure of bird populations is poorly known. Moreover, the diversity and complexity of avian social behavior, coupled with its visibility, provide an opportunity to determine how patterns of social behavior might affect genetic patterns in natural populations (Baker, 1974; 1975). Our current understanding of genetic population structure relies heavily on organisms that are relatively unknown ecologically and/ or are known behaviorally principally from studies in bottles, aquaria, and cages (fruit flies, fishes, rodents). Lack of attention to birds may partly be explained by the fact that on the whole, birds appear to be enzymatically less polymorphic than many other organisms (Powell, 1974; Manwell and Baker, 1974). Population structure and influences of selection can be examined on the basis of one or a few loci (Corbin et al., 1974; Redfield, 1973; Koehn et al., 1971), and birds may be particularly useful in studies of immigration, assortative mating, and inbreeding.

Vocal learning in white-crowned sparrows: sensitive phase and song dialects

SummaryIn a laboratory song tutoring experiment, eight male White-crowned Sparrows learned their natal song dialect, presented to them at 10 to 50 days of age, in preference to an alien dialect, presented to them from 51 to 90 days of age; two males learned the alien dialect, one developed a hybrid song, and one a simple song lacking structural detail (Fig. 3). Three other males developed aberrant songs (Fig. 5). Excepting these three aberrant songs, the song patterns of the males were similar to, though not close copies of, the tutor songs. The songs of all males remained consistent through two photoperiodically induced breeding ‘seasons’ (November–January and April–May; Fig. 4). During these ‘season’, females were induced to sing by administration of testosterone. Six females sang during the first season, eight sang during the scond season, with five singing in both seasons. Unlike those of the males, the song patterns of all females (with the exception of F9), were not similar to the tutor models. The songs of those five females that sang during both seasons remained consistent through both seasons (Fig. 6). We conclude that because the sensitive phase for song learning generally coincides with natal philopatry, White-crowned Sparrows are predisposed to learn their natal dialect which they retain through life. This result may be an important factor in the maintenance of populations structured by geographic systems of dialects.

REPRODUCTIVE BEHAVIOR OF FEMALE BUNTINGS: ISOLATING MECHANISMS IN A HYBRIDIZING PAIR OF SPECIES

The plumage characteristics of male Indigo and Lazuli Buntings are distinct, but the two species can learn each others songs. Populations comprising Indigo, Lazuli and hybrid individuals occur in the Great Plains of North America, and assortative mating has been inferred from morphometric data. We devised a laboratory assay for determining female preferences for visual and vocal characteristics of conspecific and heterospecific males and for mixtures of these characteristics, such as might be encountered in an overlap population. Females of both species gave more copulation‐solicitation displays when exposed to conspecific plumage and vocalizations than when exposed to heterospecific plumage and vocalizations. Females gave intermediate and similar responses to the combinations of conspecific plumage with heterospecific vocalizations and heterospecific plumage with conspecific vocalizations. Thus, in the absence of other potentially important variables, female reproductive behavior is consistent with the hypothesis of assortative mating, based upon both vocal and visual traits of the males and caused by female choice in this semispecies pair.