DISSERTATION RESEARCH: An Experimental Test of a New Hypothesis-Brood Parasitism as an Influence on Incubation Strategies

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Most young animals require extensive protection and nurturing by their parents while they develop. This is particularly true for young birds, because early development takes place within an egg. The parents (usually the female) must provide protection and warmth if the egg is to survive. The more time a female bird can spend sitting on her eggs, the better chance those eggs have of surviving, hatching and contributing to the next generation. However, females must also forage to get the food they need for continued health. This activity typically requires getting off the nest, thus exposing the eggs to cooling and possible predation. However, in many species, the females alleviate this tradeoff through male incubation feeding - a behavior in which the male brings food to the female, thus extending the time she can spend on the nest. Researchers have primarily considered two factors that might control patterns of incubation: the risk of nest predation, and the climate around the nest. Where potential predators are common, females should reduce the number of times they go to and from the nest, and stay on the nest as long as possible. However, because incubation feeding by males can give away the location of the nest to predators, incubation feeding should be minimal. In contrast, where climates are very cold, females should spend as much time as possible on the nest, get off only for short periods, and rely heavily on incubation feeding. However, there is a third factor that has not yet been considered, and which may cause large shifts in incubation behavior: egg removal by brood parasites, such as the brown-headed cowbird. Cowbirds and other brood parasites lay their eggs in the nests of other species, leaving these "hosts" to raise their young. Birds raising cowbirds typically raise fewer of their own young, partially because cowbirds often remove eggs from the nest of the host before or after they place their own egg in the nest. Egg removal may be more likely when an adult does not occupy the nest, thus the female should spend more time on the nest if she believes a cowbird knows the location of her nest and is thus likely to remove one of her eggs. Indeed, in previous work, the PIs found that parasitized females spend much more time on the nest than unparasitized females. The PIs will conduct tests to determine if Yellow Warblers actually shift their incubation behavior in response to the threat of egg removal by cowbirds. The PIs will use video cameras to record the incubation behaviors of unparasitized Yellow Warblers. In one random subset of nests, the PIs will attract cowbirds to the nests where they will interact with the Yellow Warblers, and then the PIs will experimentally parasitize these nests by replacing one yellow warbler egg with an infertile cowbird egg. The PIs will then film nests again to see if they have changed their incubation behaviors to protect the nest from the perceived threat of egg removal. For comparison, in another random subset of nests, after initial filming, the PIs will play the song of a Catbird, a bird which represents no threat to the nest, and mimic the disturbance caused by experimental parasitism, but the PIs will not place a cowbird egg in the nest. In these nests, the PIs will again film these nests, to determine if birds are reacting to the disturbance, rather than the parasitism itself. Finally, the PIs will also film naturally parasitized nests to compare incubation behaviors with the artificially parasitized nests. This experiment will allow the PIs to provide the first test of the importance of brood parasitism as a determinant of incubation behavior.