Other experiments have shown the high degree of plasticity in animal foraging strategies, and particular patterns of prey density can drive typically ‘risk-averse’ foragers into ‘risk-prone’ behavior. For example, in foraging choice experiments with birds and bumblebees, animals chose food with smaller but more consistently available energy.
#Acorn sccpss shared documents forms all items Patch
If animals perceive that resource levels in a patch are unpredictable in space and time, they may choose to forage in ways that minimize risk of starvation, but do not conform to classic OFT predictions. Many studies have shown that variations in resource levels can influence foraging processes, and that studies across multiple seasons/years are critical to understanding general principles of how extrinsic factors influence animal foraging movements. One important factor is the degree of spatial and temporal variation in resource availability. īoth environmental factors and intrinsic behaviors can be expected to modify an animal’s foraging strategy, and ‘optimality’ is context dependent. Thus, 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. Additionally, some foragers may have a ‘central place’ to which they must return after foraging, and this behavioral feature impacts both how they will forage and the optimality predictions. Animals may forage efficiently by either being ‘energy maximizers’ or ‘time minimizers’, and time invested in foraging can be translated into the distance a forager must move in search of resources. Much of the theory regarding foraging behavior is grounded in Optimal Foraging Theory (OFT), which predicts that a forager will maximize energy intake over time.
Such movements determine a forager’s efficiency in finding food and vulnerability to predators, both of which impact forager fitness. Understanding the factors influencing patterns of animal foraging movements is a long-standing research goal of behavioral ecology. 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. The number of trees they visit increases in years of high acorn availability, but the extra trees visited are mostly local.
ConclusionsĪcorn woodpeckers forage in a pattern consistent with optimal foraging theory, with a few fascinating exceptions of long distance movement. 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. The α-diversity values were significantly higher for Quercus lobata, but not for Quercus agrifolia, in years of high acorn production.
In 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. 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. 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. 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. Foraging movements of animals shape their efficiency in finding food and their exposure to the environment while doing so.
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