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Field work to determine possible causes of variation in avian social behavior in a nonbreeding community of 42 species was undertaken during two winter seasons in oak-juniper woodland of the Santa Rita mountains of southern Arizona. Transect observations are used to calculate monthly densities, frequency of social foraging, and size and composition of social groups for all species. Additional data taken to quantify the foraging logistics of 22 species includes foraging substrate, time spent at a foraging station, and distance travelled between foraging stations. These data are used to generate indices of logistic similarity and interspecific association for species pairs in order to test the hypothesis that species which are similar in their foraging logistics are more likely to associate than those which are not. Causes of solitary behavior and variation in intraspecific social behavior are also examined. Exclusively solitary foraging behavior is strongly associated with species that use sparse, unpredictable prey too small to share. These species, referred to as "extensive foragers," appear to be constrained from both intraspecific and interspecific association by being logistic mismatches with all but their closest competitors, with which the potential for interference competition is high. Evidence is presented to refute the hypothesis that solitary behavior is due to low vulnerability to predators. Species which are intraspecifically solitary tend to use evenly distributed resources, and gregarious species use patchy and abundant resources. Exceptions are most frequent among permanent residents, which may continue to maintain social and spacing systems used in the breeding season. Among gregarious species, permanent residents are more likely to maintain stable group sizes than winter residents. A correlation exists between similarity of foraging logistics and likelihood of interspecific association, but it is subject to considerable variance. Causes of this variance include the tendency of transient migrants not to associate with other species, and the tendency of certain species to form temporary associations, regardless of logistic similarities. Why some species form temporary ("opportunistic") associations while others do not remains unclear.

In this thesis I discuss (1) the importance of status-age variations in the foraging behavior of individuals within a flock and (2) cultural transmission of foraging information among Gray-breasted Jays (Aphelocoma ultramarina), a cooperative breeder. Results of the experiments suggest that (1) subordinates are less successful in certain foraging situations than dominants, (2) the foraging preferences of young birds are not as defined as those of adults, (3) subadults obtain foraging information from adult flock members, and (4) older subordinates may not change their established preferences as easily as dominants (or younger birds) even when a cost to that preference is introduced. I suggest that, at least in this cooperative breeder, the flock provides a valuable source of environmental information.

The breeding and feeding ecology of the Swallow, Hirundo rustica, and the Sand Martin, R. riparia, in Central Scotland is described, with particular reference to the constraints imposed by environmental conditions. The time and energy investment in reproduction was examined for all stages of the breeding season in a wide variety of environmental conditions. Brood sizes were manipulated to change the investment of each parent in the brood. The D2018 technique was used to measure flight costs (0.0848 kcal g-l h-1 for the Swallow, 0.1288 kcal g-l h-1 for the Sand Martin) and the rate at which each species collected food under a variety of conditions was examined (mean values were 0.14 assimilable kcal min-1 for the Swallow, 0.05 assimilable kcal min-1 for the Sand Martin). These data, along with measurements of the nestlings' daily energy requirements, were used to investigate energy balance and the consequences for the timing and level of each breeding attempt. It is suggested that the Swallow lays later than the Sand Martin because (a) it takes larger insects and does not lay until these become abundant and (b) the female Swallow incubates alone whereas both Sand Martin sexes incubate, hence the Swallow defers laying until the risk of encountering bad weather during incubation is low. On 65% of the days on which measurements were made Sand Martins were unable to feed adequately a brood of five or more nestlings (31% of days for the Swallow) because foraging rates were depressed by bad weather. The risk of encountering bad weather is thus likely to limit the upper level of brood size. Energetics data are used to show that parents feeding nestlings maximise the net intake rate of energy rather than that of nutrients (protein, sulphur or calcium). The Swallow's diet includes nonpreferred small items, especially when these are relatively abundant, even when the preferred large items are available - contrary to predictions of optimal foraging models. It is shown, however, that foraging effiency is high when small items are taken. In agreement with central place foraging theory the bolus size of Sand Martins is mainly determined by the distance travelled to the feeding site. This is not always the case, however, since patch and prey quality and the search methods employed have a greater influence on the bolus size of the Swallow. Overall, the study allowed the scope for breeding activity for hirundines under different conditions to be defined and showed the importance of each species' reproductive and foraging strategies in optimising the number of offspring produced.

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Presentations of three potential nest intruders were used to investigate house wren nest defense behavior, while regular nest box checks provided information regarding house wren nest-destroying behavior. During nest defense trials, male wrens were typically more active in defense than females in the egg stage but no intersexual differences existed in the nestling stage. Males' greater defensiveness could be an extension of their territory defense role. Defense vocalizations were sex biased, with males giving primarily song calls and females giving chatter calls. Both sexes maintained or lowered their defense levels from egg to nestling stage, possibly due to decreased offspring vulnerability. House wrens altered their defense behaviors among intruder species, depending on the type of threat. Males exhibited greater defense levels against the house wren model compared to the bluebird or cowbird model in the egg stage but showed no differences among model species in the nestling stage. Females exhibited no defense differences among model species. Male defense levels were affected by their breeding strategy, as attentive males were more aggressive toward the house wren and cowbird models than non-attentive males. Attentive males discriminated among intruder species while non-attentive males did not. Lack of male attentiveness, due to polygamy or attempted polygamy, appeared to be potentially costly to females, regardless of their order of pairing with the male. House wren nest destroying behavior (NDB) varied with respect to the timing of house wren nest attacks, the abandonment and reacquisition of nests following a wren attack, and the status of neighboring house wren nests at the time of a nest attack. Some supportive evidence was found for the existing NDB hypotheses, but no single hypothesis was completely supported. House wren NDB may serve several functions but may ultimately depend on house wren population density. / Department of Educational Leadership