Why Does Intensity of Avian Nest Defense Increase During the Nesting Cycle?

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Abstract
Theoretical models predict that intensity of avian nest defense should increase with age of the offspring. Empirical observations conforming with this prediction have been taken as support for these models. We here present a new explanation for the observed correlations between offspring age and level of nest defense. We propose that increased intensity of nest-defense behavior is largely a result of the methods used by the researchers who made the observations. We suggest that when an observer repeatedly visits or brings a potential nest predator to a nest, nest-defense behavior of parents is modified by positive reinforcement and loss of fear. We tested this hypothesis by measuring nest-defense behavior of female American Robins (Turdus migratorius) and male and female Red-winged Blackbirds (Agelaius phoeniceus). The tests were performed at nests visited only once during different stages of the nesting cycle and at a group of nests visited repeatedly from initiation of incubation to fledging. Nest-defense intensity (as measured by call rates, closest approach to the predator, and numbers of dives and strikes) of robins and blackbirds at nests visited once, in most cases, did not increase through the nesting cycle. At nests visited repeatedly through the nesting cycle, intensity of nest defense by both robins and blackbirds, in many cases, increased significantly. In addition to intensity of nest defense increasing at multiple-visit nests, we observed higher proportions of birds at these nests performing nest-defense behaviors than at single-visit nests.