We tested the hypothesis that long-term familiarity with neighbors is advantageous by determining whether male red-winged blackbirds (Agelaius phoeniceus) breeding adjacent to familiar neighbors have better reproductive success than other males. Using data gathered during a 10-yr study of breeding success, we found that males with familiar neighbors fledged, on average, significantly more offspring annually than males without familiar neighbors. We also found that the same males, breeding in different years on the same territories, had significantly larger harems in the years they had familiar neighbors. Improved reproductive success was due to the males' abilities to attract more females to nest in their territories. Alternative hypotheses to explain the positive relationship between familiar neighbors and breeding success were not supported by our data. Relatively high reproductive success for breeders with long-term neighbors may provide a basis for the evolution of cooperative behavior in this and other species.Individuals of many species are known or presumed to benefit from familiarity with their physical and social environments (1-4). Experience in a habitat should increase the store of information available to individuals concerning the location and quality of foraging and breeding sites and the habits of local predators. Individuals familiar with their neighbors or with other members of their social groups could potentially benefit by having prior information about, for example, dominance statuses and fighting abilities of competitors, probabilities for reciprocal actions during cooperative behaviors, and the relative quality of potential mates. These benefits would be expressed as improved survival and reproductive success, but direct evidence in natural populations of benefits from familiarity is scarce. Here we test the hypothesis that social familiarity is advantageous by evaluating the prediction that male red-winged blackbirds (Agelaius phoeniceus) with familiar neighbors produce more offspring, on average, than males lacking familiar neighbors. Social relations between adjacent neighbors are probably the most common type of interaction among territorial males of many species ofbirds. Neighbors are potential competitors for food and mates, but they are also potential deliverers of cooperative assistance. For this reason, zoologists characterize neighbor interactions as "dear enemy" relationships (5-7). Familiarity with territorial neighbors could be beneficial to males in several ways. For example, in migratory species, familiarity with previous neighbors may facilitate year-to-year territory reestablishment (8-10) by reducing time and energy allocated to disputes over boundaries. Also, for species with small contiguous territories, such as the redwing, long association among males could increase cooperation in alarm systems or in nest defense.Male redwings are sometimes aggressive toward neighbors but, as in other territorial songbirds, they also recognize them as individuals (10, 11) and...

We used data from a 12—yr study of dispersal and breeding success to investigate how female Red—winged Blackbirds (Agelaius phoeniceus) make decisions about movements between nests, both within and between years. Females were studied on eight separate marshes that were sufficiently close to permit inter—marsh movement. Our analyses showed that (1) a female's previous success affects her subsequent nesting decision, especially between years; (2) females exhibit strong marsh fidelity; and (3) the presence of familiar male neighbors affects female movement decisions. Little support was found for the ideas that females move in response to different types of nest predators, familiarity with current mates, or nesting success of other females on their marshes. Our results suggest that (1) there is more social constraint on changing territories and marshes, especially within breeding years, than previously suspected, and (2) although mate fidelity is relatively weak and unimportant, male "neighborhoods" or "breeding groups" affect female nesting success and, hence, influence movements.

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