Abstract
BackgroundForaging movements of animals shape their efficiency in finding food and their exposure to the environment while doing so. Our goal was to test the optimal foraging theory prediction that territorial acorn woodpeckers (Melanerpes formicivorus) should forage closer to their ‘central place’ in years of high resource availability and further afield when resources are less available. We used genetic data on acorns stored in caching sites (granaries) and adult trees for two oak species (Quercus lobata and Quercus agrifolia) to track acorn movements across oak savanna habitat in central California. We also compared the patterns of trees these territorial bird groups foraged upon, examining the effective numbers of source trees represented within single granaries (α), the effective number of granaries (β), the diversity across all granaries (γ), and the overlap (ω) in source trees among different granaries, both within and across years.ResultsIn line with optimal foraging theory predictions, most bird groups foraged shorter distances in years with higher acorn abundance, although we found some exceptionally long distance foraging movements in high acorn crop years. The α-diversity values were significantly higher for Quercus lobata, but not for Quercus agrifolia, in years of high acorn production. We also found that different woodpecker family groups visited almost completely non-overlapping sets of source trees, and each particular group visited largely the same set of source trees from year to year, indicating strong territorial site fidelity.ConclusionsAcorn woodpeckers forage in a pattern consistent with optimal foraging theory, with a few fascinating exceptions of long distance movement. The number of trees they visit increases in years of high acorn availability, but the extra trees visited are mostly local. The territorial social behavior of the birds also restricts their movement patterns to a minimally overlapping subsets of trees, but the median movement distance appears to be shaped more by the availability of trees with acorns than by rigid territorial boundaries.
Highlights
Foraging movements of animals shape their efficiency in finding food and their exposure to the environment while doing so
Our objectives were to: (1) compare foraging movements across years by tracking distances that birds moved acorns from source trees to granaries for multiple family groups in two oak species; (2) assess effective numbers of source trees visited by different social groups (α-diversity), the effective number of granaries used by the bird groups (β-diversity), the accumulated number of seed trees foraged upon for the entire study site (γ-diversity), and the extent to which different social groups foraged on the same or different source trees (ω-overlap), both within and across years
We found variation in overlap from 0–1 among the family groups sampled in both years, and higher average overlap in source tree use between years for Q. lobata (ω02–04 = 0.71) than for Q. agrifolia (ω06–07 = 0.41), supporting other work asserting Q. lobata is the preferred food item of the two oak species [44]
Summary
Foraging movements of animals shape their efficiency in finding food and their exposure to the environment while doing so. Understanding the factors influencing patterns of animal foraging movements is a long-standing research goal of behavioral ecology. Such movements determine a forager’s efficiency in finding food and vulnerability to predators, both of which impact forager fitness. Full list of author information is available at the end of the article energy intake over time [1,2,3]. This optimal model can take many forms. If an animal is foraging optimally and must return to a central place, OFT predicts that it will forage in patches close to that central place, unless the energy gained from a more distant patch is higher than the energy expended in travel [3,9]
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