The dusky orange-crowned warblers (Oreothlypis celata sordida) found on Santa Catalina Island, California are a little different than their brethren. Most orange-crowned warblers build their nests on or close to the ground. But not the dusky orange-crowned warbler.
A study performed by scientists from the University of Glasgow, Colorado State University and the Smithsonian Migratory Bird Center found that these interesting little birds build their nests high and low on the island.
There isn't much competition from other birds on the island for the kind of nesting site the warbler requires. This lack of competition from other birds gave the team of scientists a unique opportunity to study the birds' selection for nesting sites in a setting where an unusually wide variety is available.
The scientists found that in this uncompetitive environment the dusky orange-crowned warbler seems to prefer coastal sagebrush in the drier, or xeric, habitat and lemonade-berry in the moderately moist, or mesic, habitat on the island. Not surprisingly, these also happen to be the most common types of vegetation found in the 2 habitats.
The birds might simply be choosing a potential nest site based upon what is readily available. Or they may be choosing to build nests in areas where there are many similar sites nearby, making it harder for predators to locate the nests.
Over the course of the study, the team found 56 nests in the habitat area, and a significantly large number of the nests—almost 75%—were built off the ground. The dusky orange-crowned warblers most likely build their nests higher to avoid the most common predators on Santa Catalina Island—snakes, foxes, and rats.
There is little threat from animals that prey on elevated nests, since typical predators such as jays don't live on the island. The study also found the elevated nests were bigger than the warbler nests found on the ground, but this is probably due to the fact that a nest in a tree requires more structural support. The team also discovered the birds usually place the entrances to their nests pointing downhill—most likely as an additional escape strategy from predators.
The results of this study are the first to provide detailed information about the nesting habits of this subspecies of warbler. We hope future research will reveal even more about their unique nesting habits and how these choices relate to their continued survival.
This article summarizes the information in this publication:
Montag, Hannah, Nager, Ruedi, Ghalambor, Cameron K. and Sillett, Scott. "Nest Site Selection of the Endemic "Dusky" Orange-crowned Warbler (Oreothlypis celata sordida) on Santa Catalina Island". In: Gardelon, D. K., Proceedings of the 7th California Islands Symposium., pp. 283-291. 2009.
The sordida subspecies of orange-crowned warbler (Vermivora celata) often nests off-ground, unlike mainland populations. Here, we quantitatively describe nest site selection during one breeding season in two contrasting habitats, xeric and mesic, to better understand adaptive nest site selection. As expected, the Catalina Island orange-crowned warbler exhibited a high level of plasticity in nest site location with larger variation in nest heights when compared with mainland subspecies. We found that only canopy cover above nests was statistically different between the xeric and mesic habitats, being lower in the xeric habitat. There was also a wider variety of substrates utilized in the mesic habitat, which was likely related to differences in substrate diversity. There was a significant preference for a northeast orientation of nests and nest entrances in the mesic habitat, but a preference for nest entrances to be orientated west on the xeric plot. Nest and nest entrance orientation were highly correlated with the aspect of the slope on which the nest was situated.
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