Breeder age, group size, and fledgling production in the cooperatively breeding Red-cockaded Woodpecker (Dryobates borealis)

Abstract

Field studies of the factors that influence reproductive success are crucial to life history theory. We investigate annual fledgling production by the monogamous, cooperatively breeding Red-cockaded Woodpecker (Dryobates borealis) in relation to breeder age, experience, and group size at Brosnan Forest, South Carolina, and evaluate several associated hypotheses. Productivity in both female and male breeders increased nonlinearly with age, peaked at 6 and 7 years, respectively, and then declined. We tested if the early increase in productivity with age was associated with age, per se (general skills), or breeding experience. We found for both males and females that the productivity of novice breeders increased significantly with age, indicating age effects independent of prior experience, but that experienced breeders were no more productive than novices the same age. We conclude that the basic skills that accrue with age, but not prior breeding experience, explain increasing success among young birds. The probability of a group nesting increased nonlinearly with female and male age, reaching 100% among females ≥ 7 and males ≥ 11 years old. We found support for both the constraint and restraint hypotheses for young males, but only for the constraint hypothesis among young females. Declining productivity with senescence counters the restraint hypothesis's prediction of increased effort late in life, while the maximal propensity to nest among older birds supports it. Differential survival failed to explain increasing productivity with age. Holding breeder ages constant, we found a linear effect of group size on annual fledgling production, with productivity increasing by 12% with each added helper through the largest group sizes. This suggests (1) no evidence of net compensation as group size increased, (2) no evidence of negative feedback from reproductive or resource competition in large groups, and (3) inclusive fitness is a selective force in helping.