Acorns, insects, and the diet of adult versus nestling Acorn Woodpeckers

The diet of Acorn Woodpeckers (Melanerpes formicivorus) in central coastal California consists of a mixture of high-quality (insects) and low-quality (stored acorns) food during the spring breeding season. We used stable isotope ratios obtained from blood samples to estimate possible differences in the proportion of these items being fed to nestlings and being consumed by breeding adults, as well as the extent to which the diet of nestlings shifts between insects and acorns during the nestling period. Based on both feeding observations and items recovered from nestlings, older nestlings were fed 28% acorns. Using this value as a baseline for the isotope analyses, the diet of breeding adults was estimated to consist of 90% acorns. Based on repeated samples from the same nestlings over time, the estimated proportion of acorns in their diet increased from 19% in 9- to 12-d-old nestlings to 42% in 23- to 26-d-old nestlings. There was a significant correlation between the isotope values of adults and the nestlings they fed, indicating that different groups of Acorn Woodpeckers have significantly different diets. Although important for successful reproduction in this species, stored acorns are not the primary food resource used to feed young. Instead, the availability of stored acorns allows adult Acorn Woodpeckers to provide nestlings with more protein-rich insects while maintaining themselves on relatively protein-poor, low-quality acorns. Differences in the diets of adults and their nestlings are likely widespread among species that feed, at least in part, on low-quality foods, such as fruit and seeds, during the breeding season.