AUTUMN 2000/VOLUME 14, NUMBER 4

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Cerulean Warbler Atlas Project

Canadian researchers provide surprising insights into the breeding biology of this important species

Throughout much of its breeding range, the Cerulean Warbler is experiencing precipitous declines in numbers; estimates derived from Breeding Bird Survey data show as much as a 3.4 percent decline per year from 1966 to 1997. In Canada, the Cerulean Warbler has been designated as "vulnerable" by the Canadian Wildlife Service.

In general, Cerulean Warblers are thought to require large tracts of mature, deciduous forest; however, their small-scale habitat affinities are poorly understood beyond the general conclusion that they prefer large trees with dense canopies as nest habitat.

In sharp contrast to continental trends, recent research indicates that eastern Ontario is home to a sizable population of Cerulean Warblers that is experiencing high reproductive success.

The overall goal of the research by our group at Queen's University is to explore the demographic, behavioral, and ecological factors regulating the Cerulean Warbler population in eastern Ontario. Much of our research on the breeding grounds is undertaken in the Lake Opinicon region of eastern Ontario on or near the property of the Queen's University Biological Station. Average nest success from 1994 to 1997 was 76.2 percent, with an average clutch size of greater than three eggs. During these years, the birds suffered low nest predation rates, and scant evidence existed for brood parasitism by Brown-headed Cowbirds.

In January 1998, however, the largest ice storm in Canadian history swept through our area and drastically affected the breeding success of our study population. More than 60 percent of the habitat used by Cerulean Warblers in our study area suffered severe damage.

In May 1998, returning birds attempted to nest in areas similar to those used in previous years: that is, they attempted to select nest sites with similar heights and with similar foliage cover characteristics. Nevertheless, nesting success in 1998 dropped to 8 percent. The loss of canopy foliage could result in a decrease in nesting success for several reasons: alteration of nest microclimate, increased conspicuousness of the parents around the nest site, and the direct and indirect effects of changes in insect prey populations.

After conducting experiments and performing detailed behavioral watches at nest sites in 1998 and 1999, we concluded that the most likely cause of decreased nesting success in 1998 was a reduction in insect prey resulting from ice storm damage to vegetation. A decrease in insect prey could affect Cerulean Warblers directly, by decreasing food availability for nestlings, and indirectly, by causing a dietary shift in Blue Jays from insects to young birds. This story, however, does appear to have a happy ending. Reproductive success in 1999 rebounded toward prestorm levels, and this resurgence in breeding success was accompanied by a significant shift in nest-site selection patterns. This shift carried over into the spring of 2000, and reproductive success continued to climb toward prestorm levels. This relatively rapid shift in behavior implies that Cerulean Warblers possess a degree of plasticity in their habitat affinities and that this plasticity rendered the population relatively resilient to the particular disturbance. This apparent resilience bodes well for the future health of the Cerulean Warbler in eastern Ontario.

We want to thank everyone who contributed to this research during the last seven years, especially volunteers from the Kingston Field Naturalists, the Rideau Valley Naturalists, and all the landowners in eastern Ontario who allowed us to investigate their land in search of these wonderful birds.

Jason Jones is a doctoral candidate in the Department of Biology at Queen's University who has worked with Cerulean Warblers for five years. Jennifer Barg is a master's degree candidate in the same department and has worked with Cerulean Warblers since 1998. Raleigh J. Robertson is a professor at the Department of Biology, Queen's University, Kingston, Ontario, Canada.