Costs and Benefits of Joint Nesting in the Acorn Woodpecker

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Abstract
The acorn woodpecker (Melanerpes formicivorus) is a cooperative breeder that lives in permanently territorial family groups of 2-15 individuals. Although most groups contain only one breeding female, 22% contain two jointly nesting females, which both lay eggs in the same nest and cooperatively rear the young of that nest. Both joint and solitary nesting coexist within the same California [USA] population, even though females that nest jointly destroy each other''s eggs and have significantly lower annual reproductive success than females that nest singly. Here we examine the relative costs and benefits of joint nesting, primarily by means of a demographic model. Our analysis indicates that a number of ecological and social factors have influenced the relative advantages and disadvantages of joint nesting gand that the fitness of females that nest jointly appears to be comparable to (or only slightly lower than) tht of females that breed singly. Factors reducing the short-term reproductive costs of joint nesting include the absence of strong reproductive bias between jointly nesting females, the limited availability of independent-breeding opportunities, and the increased probability of such females'' acquiring and maintaining tenure in a breeding territory. Higher annual survival probabilities of jointly nesting females may play an important role as well, although this potential benefit of joint nesting is not statistically significant.