Predation is a major force driving the evolution of animals. For birds, the greatest risk of predation occurs during the nesting season when a great variety of animals prey on eggs and nestlings.
Among the many predators are: raccoons, snakes, foxes, and other birds. So birds must exercise great care in choosing their nest sites.
Each kind of bird seems to have its preferred nesting strategy. For example, woodpeckers always build their nests in tree cavities and grebes always build a floating nest.
But can birds change their nesting strategy?
The orange-crowned warbler is a small bird that nests all across North America, from Labrador to Alaska and south through the western United States to southern California.
In the fall, this insect-eater migrates to the warmer areas of the southern United States and Mexico. Across its entire breeding range, consisting of thousands of square miles, it always builds its nest on the ground in dense vegetation.
Except on Catalina Island.
Catalina Island is part of the Channel Island group and is located about 20 miles off southern California.
Only here do orange-crowned warblers often build their nests in trees.
National Zoo scientists Susana Peluc and Scott Sillett set out to investigate why the nesting strategy would be different on this island.
They set up a study site in the steep canyons of the island, a favorite haunt of the warblers.
Orange-crowned warblers are quite abundant in the vegetated canyons on the island.
The island is steep and rocky with peaks rising 2000 feet above sea level.
Open grassy areas are frequented by introduced herds of buffalo, deer, and goats.
But in the canyons there is a dense overstory of evergreen oak trees and a lush ground layer of vegetation that is very appealing to the warblers.
Some warblers build their nests in the trees, while others nest on the ground.
The incentive for nesting in trees is that more young fledge in tree nests as compared to ground nests.
Tree nests are so successful because Catalina Island has no scrub jays, a relentless egg-stealer on the mainland and on nearby Santa Cruz Island.
Jays are adept at finding nests among tree branches and eating the eggs or young.
Nests built on the ground, and concealed among grasses and other vegetation, are safer from the jays.
But do warblers on Catalina Island even know what a jay is?
Are they building nests in trees because they know there are no jays around?
Susana and Scott designed an experiment to test the warblers.
Females that were looking for nest sites were divided into three groups: some were presented with a model of a scrub jay, and the stuffed bird was moved frequently, and taped jay calls were played.
Other females were presented with a stuffed house finch, a warbler-sized bird that eats seeds. The third group was the control group, they were left alone.
All the females that had been terrorized by the dummy jay built their nests on the ground while females in the other groups often nested in the trees.
Not only did the warblers recognize the jay as a predator, but they shifted their nesting strategy in response.
In 2005, research will also be conducted on another member of the Channel Islands, Santa Cruz.
This island has jays, the endemic Santa Cruz scrub-jay, and also has numerous feral pigs, which have decimated the vegetation at the ground level.
So the warblers here get a double whammy: efficient predators of nests in trees, and little cover in which to hide their nests on the ground.
Not surprisingly, warblers are not very numerous on Santa Cruz Island.
Efforts are currently underway to control the pig overpopulation, and scientists will be studying the warblers to see how they respond.
This article summarizes the information in this publication:
Peluc, Susana I., Sillett, Terence Scott, Rotenberry, John T. and Ghalambor, Cameron K. 2008. Adaptive phenotypic plasticity in an island songbird exposed to a novel predation risk. Behavioral Ecology, 19(4): 830-835.
Avian nest site selection and levels of parental care require assessments of numerous fitness costs and benefits. Nest site selection in open cup-nesting species is considered a relatively conservative trait; most species and genera are confined to nesting witliin particular vegetation strata. The nesting stratum further determines risk to nest prédation, the principal cause of reproductive failure. We document predator-induced plasticity in nest site placement and levels of parental care in orange-crowned warblers (Vermivora celata) on an island lacking avian nest predators. We show a shift from ground nesting, characteristic of mainland populations, to off-ground nesting that appears adaptive relative to higher prédation levels of ground nests. By altering the perceived nest prédation risk via experimental introduction of a model avian predator prior to nest building, we demonstrate that warblers shift nest sites to more concealed ground locations. Moreover, warblers differentially adjust nest visits to feed nestlings in the presence of the predator: reducing feeding more at less concealed off-ground nests than at more concealed ground ones. Both shifts in nest site placement and feeding rate adjustments suggest adaptive phenotypic plasticity in response to increased perceived prédation risk, providing evidence that birds continuously assess variation in the fitness costs and benefits of their behavioral decisions. Key words: adaptive phenotypic plasticity, nest site selection, parental care, prédation risk.
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