Behavioral interactions between red-winged blackbirds and long-billed marsh wrens and their role in the evolution of the redwing polygynous mating system

Behavioral interactions between red-winged blackbirds and long-billed marsh wrens are an important selective force influencing redwing reproductive tactics- Marsh wrens, which destroy redwing eggs and kill redwing nestlings, were the most important nest mortality factor in my redwing population and possibly also in other marshes throughout the range of sympatry between these species. This nest mortality factor probably selected for four different adaptations through which redwings reduce the impact of marsh wrens on their nesting. First, redwings of both sexes aggressively exclude marsh wrens from the vicinity of their nests. Second, redwings avoid marsh wrens by breeding in sparser vegetation which is more easily defended against wrens. Marsh wrens, on the other hand, seem to prefer denser vegetation where they are more likely to avoid redwing aggression. These differences in habitat selection by redwings and marsh wrens result in a spatial segregation of their breeding areas. Third, female redwings reduce the impact of marsh wrens by clumping their nests and hence improving their nest defense through cooperation with their neighbors. Fourth, females preferentially join older males and harems with older females probably because such older birds are more efficient in nest defense against marsh wrens. Breeding in a harem with experienced birds increases a female's chance of reproductive success. Redwings exhibit two forms of polygyny. True harem polygyny has probably evolved as a compromise between two selective forces: (1) high nest predation rates which drive the evolution of the clumping tendency by females; and (2) seasonally abundant, predictable, and relatively uniformly distributed food resources in marshes and nearby uplands, which presumably favor the evolution of territoriality. As a result, females breed colonially within male territories. The most important prediction from this hypothesis is that harem size selected for in various populations should be correlated with the intensity of predation. This is because females should be selected to adjust their degree of clumping and hence their degree of cooperation in nest defense to the average nest predation pressure. This prediction is supported by data from various redwing populations throughout North America. In addition, redwings also exhibit resource defense polygyny. The presence of older experienced birds appears to be the most important factor responsible for a great degree of variation in the size of a harem attracted to individual male territories. Thus territories with older experienced males and females attract large harems, whereas those without experienced birds acguire small harems. Preferences of females for older males and for harems with older females could be explained in terms of a greater contribution to nest defense by such experienced birds as compared with young, inexperienced individuals- The pattern of distribution of older experienced individuals in the marsh, therefore, determines the mating pattern in a breeding redwing population. The distribution of older experienced individuals of both sexes is determined by their strong tendency to return to the same territories year after year. Consequently, mating success of newly established males is initially determined by the number of females acquired by previous territory holders and their overwinter survival rates. These observations indicate that mating success of any given male is a function of: (1) the number of old, experienced females breeding on his territory; (2) attractiveness of his territory to new females as reflected by the number of older females; and (3) attractiveness of his territory to new females in terms of his age-related experience. Evidence from this study suggests that to explain the adaptive value of redwing polygyny, we have to consider both the cooperative and competitive interactions between females. A model combining adaptive values of true harem polygyny and resource defense polygyny is presented.

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