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

-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 multiplevisit nests, we observed higher proportions of birds at these nests performing nest-defense behaviors than at single-visit nests. Received 19 March 1985, accepted 20 October 1985. MOST studies of nest defense in altricial birds have revealed increases in the intensity of nest defense by parent birds through the nesting cycle (Smith 1950; Erpino 1968; Barash 1975; Curio 1975; D'Arms 1978; Weatherhead 1979, 1982; Andersson et al. 1980; Greig-Smith 1980; Patterson et al. 1980; Biermann and Robertson 1981; East 1981; Blancher and Robertson 1982; R6ell and Bossema 1982; Merritt 1984; Shields 1984). Two explanations have been offered for the temporal changes in the intensity of nest defense. The first, and currently most popular, explanation is derived from the theory of parental investment (Trivers 1972). Barash (1975) extended the theory to include parental defense of eggs and young. Whether based on cumulative past parental investment (Trivers 1972, Barash 1975) or future expected benefits minus expected costs (Dawkins and Carlisle 1976, Boucher 1977, Maynard Smith 1977), this hypothesis predicts an increase in nest defense as the young near fledging. In a similar fashion, Andersson et al. (1980) hypothesized that the relative difference between the expected 1 Present address: Institute for Environmental Studies, University of Washington, Seattle, Washington 98195 USA. survival of parents and their young decreases as the nestlings grow older, making parents willing to defend older nestlings more aggressively. The second explanation is based on the hypothesis that older nestlings are more conspicuous to predators. The nest and its contents become more conspicuous as the nesting cycle progresses, necessitating an increase in the intensity of nest defense to counteract the nest's increased conspicuousness to predators (Skutch 1949, Harvey and Greenwood 1978). We offer a third, alternative explanation for increases in nest defense through the nesting cycle. We propose that the increases are largely the results of the methods used by the researchers who made the observations. We suggest that when an observer repeatedly visits or brings a potential predator to a nest and records the parent birds' responses, the nest-defense behavior is gradually modified by positive reinforcement and loss of fear. Positive reinforcement is involved because the parent birds have been rewarded repeatedly for their nest-defense behavior. After being attacked, the observer or other potential nest predator leaves without harming the nest. A loss of fear is involved because the defending birds gradually 318 The Auk 103: 318-327. April 1986 This content downloaded from 157.55.39.49 on Mon, 29 Aug 2016 05:19:04 UTC All use subject to http://about.jstor.org/terms April 1986] Changes in Nest-defense Intensity 319 learn that the observer or other potential nest predator is not dangerous to them; however, they still view the object as a threat to the nest's contents. Both positive reinforcement and loss of fear could explain much of the reported increases in nest-defense behavior. We examined these explanations by measuring nest-defense behavior in experiments with American Robins (Turdus migratorius) and Redwinged Blackbirds (Agelaius phoeniceus). STUDY AREA AND METHODS Nests of blackbirds and robins were located within the city of Madison, Wisconsin. Some nests were visited every 3 days, whereas others were visited only once. Blackbird nests visited once were located in Nielsen, University Bay, Redwing, and Odana marshes; nests visited repeatedly were located in Kettle Marsh. These areas are open water, cattail (Typha spp.) marshes (Bedford et al. 1975). At no time during the study period were humans seen in any of these marshes. All robin nests were located on the University of Wisconsin campus and adjacent neigh-