Daniel B. Thompson

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.

ALLOZYME FREQUENCIES IN A LINEAR SERIES OF SONG DIALECT POPULATIONS

Evaluating the biological consequences of avian song dialects is important in at least two ways. First, where appropriate experimental evidence is available, we know that dialect markers in song are learned (Marler, 1970; Marler and Mundinger, 1971). Dialect systems thus offer an opportunity to determine if and how culturally transmitted vocal signals, a fundamental feature of many songbird societies, influence patterns of gene flow. Second, despite much fine descriptive work, few attempts have been made to connect the field of behavioral research on song dialects to that of evolutionary genetics. The central issue is whether or not population specific vocal signals (dialects) cause mating to take place primarily among members of the same dialect population. This question does not specify any particular behavioral mechanism that might be hypothesized nor should it suggest that gene migration is an all-or-none phenomenon. Gene flow between adjacent dialects may vary in amount or direction depending upon such possible influences as dialect discrimination affecting mate selection, recency of contact of the populations (Remington, 1968), availability of unoccupied breeding habitat, or major environmental disturbances. If song dialects reduce gene migration among dialect populations, however, then it is reasonable to predict that neighboring dialect populations may exhibit genetic differences (Templeton, 1980a). The divergence may occur because of founder effect, genetic drift, or because of selective differences in occupied habitats as when, for example, dialects occur along an environmental gradient (Endler, 1977). differential coadaptation of genomes may also be involved. Possible outcomes of the interaction of gene flow, environmental gradients, magnitude of selection pressures, and population size have only recently begun to yield to theoretical analysis (Endler, 1977; Slatkin, 1978; Lande, 1980) and we hope that our studies may be of some use in this regard. Our field data, while not constituting a test of this developing theory, may serve to underscore the significance of gene flow in population differentiation. The four song dialect populations of Whitecrowned Sparrows treated here extend over a total of only 35 km distance in an environment of apparent homogeneity in climate and vegetation. In view of the potentially high vagility of these birds, the possible consequences of discontinuities in gene flow caused by social structure may be more easily discovered. In this report we provide allozyme frequency data from four dialect populations occurring in a linear sequence along the central California coast. From these data we describe the nature and extent of genetic differentiation within and among the four song dialect populations to test the hypothesis of genetic differentiation of dialects.

The role of male vs male interactions in maintaining population dialect structure

SummaryNonmigratory populations of Whitecrowned Sparrows in coastal California exhibit dialects in territorial male songs that are stable in space and time. By field playback experiments, we tested a prediction from the hypothesis that male aggressive interactions prohibit mixing of song dialects. Playback of the home dialect ‘Clear’ song to territorial males singing the ‘Clear’ dialect resulted in less response than that given to playback of the neighboring ‘Buzzy’ dialect. Response to both ‘Buzzy’ and ‘Clear’ dialect songs by target ‘Clear’ males, however, was greater than that given to the ‘Bodega’ dialect recorded 55 km away (Figs. 2 and 3). We conclude that these results are consistent with the hypothesis that male-male aggressive interactions play an important role in reducing dialect mixing.

DIFFERENT SPATIAL SCALES OF ADAPTATION IN THE CLIMBING BEHAVIOR OF PEROMYSCUS MANICULATUS : GEOGRAPHIC VARIATION, NATURAL SELECTION, AND GENE FLOW

Patterns of geographic variation in tree‐climbing ability of Peromyscus maniculatus were used to examine the influence of spatial variation in natural selection and gene flow on the genetic divergence of climbing behavior among populations. Offspring of adults of two subspecies sampled from 10 localities in montane conifer forest, conifer woodland, and desert scrub/grassland habitats were raised in the laboratory and tested to determine their tree‐climbing ability (the maximum diameter artificial rod that a mouse could climb). Comparisons of mean rod‐climbing scores revealed that individuals of P. m. rufinus sampled from montane conifer forest and conifer woodland in Arizona were better climbers than P. m. sonoriensis sampled from conifer woodland and desert habitats in Nevada. This result was consistent with the hypothesis that natural selection has produced large‐scale adaptation in climbing behavior. However, the climbing ability of P. m. sonoriensis sampled from conifer woodland habitats on isolated mountaintops in Nevada has not evolved in response to natural selection to the degree expected. In addition, populations sampled from desert grassland habitat, adjacent to woodland P. m. rufinus in Arizona, have climbing abilities that are not significantly different from conifer woodland populations. These observations indicate that local adaptation was constrained. An estimate of the heritability of climbing ability (h2 = 0.352 ± 0.077) indicates that lack of a response to selection was not due to the absence of additive genetic variation. In addition, regressions of interpopulation differences on the degree of geographic isolation between pairs of populations do not support the hypothesis that gene flow between habitats has constrained evolution. Instead, a combination of historical events and insufficient time to respond to selection appears to have influenced geographic variation and the spatial scale of adaptation in climbing ability.

Seed selection by dark-eyed juncos (Junco hyemalis): optimal foraging with nutrient constraints?

SummaryObservations of the foraging behavior of six captive dark-eyed juncos (Junco hyemalis) are used to test the assumptions and predictions of optimal diet choice models (Pyke et al. 1977) that include nutrients (Pulliam 1975). The birds sequentially encountered single seeds of niger thistle (Guizotia abyssinica) and of canary grass (Phalaris canariensis) on an artificial substrate in the laboratory. Niger thistle seeds were preferred by all birds although their profitability in terms of energy intake (J/s) was less than the profitability of canary grass seeds. Of four nutritional components used to calculate profitabilities (mg/s) lipid content was the only characteristic that could explain the juncos seed preference. As predicted by optimal diet theory the probability of consuming niger thistle seeds was independent of seed abundance. However, the consumption of 71–84% rather than 100% of the seeds encountered is not consistent with the prediction of all-or-nothing selection. Canary grass seeds were consumed at a constant rate (no./s) independent of the number of seeds encountered. This consumption pattern invalidates a model that assumes strict maximization. However, it is consistent with the assumption that canary grass seeds contain a nutrient which is required in minimum amounts to meet physiological demands (Pulliam 1975). These experiments emphasize the importance of incorporating nutrients into optimal foraging models and of combining seed preference studies with studies of the metabolic requirements of consumers.

Behavioral consequences of song learning: Discrimination of song types by male white-crowned sparrows

Abstract The functional significance of learned population differences in male song in the white-crowned sparrow was explored in natural populations using playback tests. Laboratory results have shown that learning of the population-specific song seems to take place in early life and is strongly dependent upon the nature of the auditory experience at that time. However, the varied results of recent studies make it difficult to reach a confident conclusion about the ecological functions of song learning. The present research took advantage of naturally occurring variation in the differences between songs of adjacent populations to determine a function relating degree of difference in song to intensity of territorial singing elicited. Applying a typological evaluation of syllable structure to the four segments of the song allowed a crude quantitative ranking of the differences between local songs and playback stimuli. These results, together with those of other studies, suggest a unimodal aggressive response function of males to songs of other males. A maximum response to songs slightly different from the local song environment suggests that male exclusion based upon acquired song components may contribute to the maintenance of discrete and stable song dialects.

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