Date of Award:

5-1-1975

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Biology

Committee Chair(s)

Keith L. Dixon

Committee

Keith L. Dixon

Committee

Carl D. Cheney

Committee

LeGrande C. Ellis

Committee

Ivan G. Palmblad

Committee

Allen W. Stokes

Abstract

The purpose of this study was to examine some aspects of winter social behavior in Dark-eyed Juncos (Junco hyemalis) in an effort to understand the accommodation which members of a flock must make between the need to compete for limited resources and the need to mediate resultant aggression. The specific objectives of the investigation were: (1) to analyze the form and function of visual and auditory signals used by juncos in a social context; and (2) to determine quantitatively the relationship between agonistic behavior and each of several factors including season, time of day, sex, rank, degree of familiarity between individuals, and resource dispersion. The research was conducted on juncos of a migratory population wintering in the vicinity of Logan, Utah. Data were collected from free-living flocks from 1972-75 and from six, six-member flocks confined in large, outdoor aviaries during the winter of 1973-74. Behavioral information was obtained using systematic, observational procedures supplemented by movie films and tape recordings. Wintering juncos communicated with one another through a variety of visual and vocal behaviors. Although two vocalizations functioned expressly to attract conspecifics, the majority of junco signals served primarily to space birds within a flock. Juncos maintained individual distances through attack, escape, and display. Attack was characterized by orientation toward the rival, a horizontal body posture, an extended neck, sleeked feathers, and movement toward the rival. Escape was typified by elements incompatible with those of attack. Visual displays contained elements both of attack and of escape together with elements which exposed conspicuous structures or which increased the displayer’s apparent size. Vocal displays consisted of at least four calls which functioned as distance increasers and which were associated commonly with visual displays but only rarely with attack or escape. Three broad categories of junco signals were distinguished on the basis of their meaning to the bird giving the cue: (1) incidental signals -- cues provided by an individual which was not directing its attention toward a respondent; (2) associative signals -- cues given by an attacking or escaping junco which communicated the actor’s intent to nearby conspecifics; and (3) purposive signals or displays behaviors which had the specific purpose of conveying information to conspecifics. Heavy reliance upon incidental signals by flock members reflected smooth functioning of the social system, whereas the use of purposive signals appeared to indicate the reverse. It was hypothesized that some junco signals may serve more than one function; for instance, a threat vocalization given by a junco upon arrival at a resource might act as a resource location cue to conspecifics. The frequency and form of agonistic interactions among flocked juncos varied with several factors. The frequency of encounters among captives increased as the winter progressed. Counteracting this seasonal trend was an apparent tendency for subordinate birds to avoid dominants, as evidenced by (1) an increase in mean distances between flock members from midwinter to early spring and (2) a proportionately greater seasonal increase in low intensity encounters than in high intensity interactions. The frequency of agonistic encounters among captive juncos additionally varied through the day, peaking in the morning and in the evening. Diurnal variations in agonistic behavior tended to be positively associated with those of activities which promoted competition, close proximity between birds, or a high state of arousal. Mean distances between flock members were greatest when agonistic activity was high, suggesting that subordinates avoided dominant flock-mates during times of high agonistic activity, diurnally as well as seasonally. Such behavior by subordinates should have promoted smooth functioning of the social system through the day and through the winter. Among the captive juncos, males were dominant to females. Males were more aggressive than were females both in midwinter and in early spring, and they showed a seasonal increase in aggressiveness whereas females did not. Males and females differed in the form of encounters which they initiated, in that only males pursued other juncos in flight. Social rank was associated positively with aggressiveness in captive juncos in midwinter. This relationship was less strong in early spring because subdominant males showed a greater seasonal increase in aggressiveness than did high-ranking males. March testes weights of captive males were positively associated with social rank although not with aggressiveness. Experiments in which captive individuals were transferred for 24-hour periods from their home pens to: another pen containing a resident flock showed that strangers participated in a significantly higher proportion of high intensity, agonistic encounters than would be expected by chance. Most strangers became subordinate to all members of the resident flock. The frequency of high intensity, agonistic encounters among free-living juncos increased exponentially with increases in bird density at a food source. Equivalent numbers of juncos showed more aggressive behavior at concentrated food sources than they did at food uniformly distributed over a relatively large area. A greater number of flock members were able to feed simultaneously when food was scattered over a large area, than when the same amount of food was concentrated in a small area. The social behavior of wintering juncos was characterized by two ostensibly opposing behaviors: gregariousness and intolerance. Behavioral conflict was not strongly evident, however, because the point at which conflict between gregariousness and intolerance was greatest occurred where the strength of both responses was low. Behavioral conflict generated by competition for resources was more powerful than that created by gregariousness and intolerance but was ameliorated by a social system which maintained space between flock members by convention. Subordinate birds recognized and voluntarily maintained the distance of intolerance of dominant flock-mates. This subordinate-controlled system operated well in established flocks with adequate resources. The social system of flocked juncos may be of particular benefit in patchy environments. When the birds come together to exploit clumped resources, they are familiar with one another, and their behavior is constrained by their respective social positions. The only way for birds to maintain the familiarity necessary for a system based upon social rank is to live in flocks. The costs of flocking may exceed the benefits thereof when resources become scarce or when flock size is large. Subordinate birds, usually females, are those most likely to suffer when conditions are adverse. Competition and gender-related dominance may be responsible for a sex ratio of 81 males:19 females found among migratory juncos wintering in the vicinity of Logan. Intersexual competition for essential resources is one of several factors which might exert selective pressure upon females of this species to migrate to wintering areas not occupied by males.

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