From 1997 to 2001, the Canadian Wildlife Service (CWS) of Environment Canada studied forest bird productivity in the Halton Region. Breeding populations of Wood Thrush and Rose-breasted Grosbeak were monitored in forests ranging from 20 to 1300 hectares (Colling Tract, Crawford Lake Conservation Area, Kelso Conservation Area, Limestone Hall, Mount Nemo Conservation Area and the Hilton Falls Agreement Forest). The project was implemented through a partnership with the ONE Monitoring Program, who secured funding for field staff.

In 2000-01, CWS mainly focused on monitoring return rates, reproductive success, brood-parasitism and multiple nesting attempts for Wood Thrush. Wood Thrushes will only nest in adequately forested areas with a fairly specific habitat structure. Therefore, they are a good indicator of forest health and function.

Forests act as habitat "sources" for birds when breeding females successfully produce enough young to sustain the populations. "Sinks" are habitats in which annual mortality exceeds fecundity. Understanding the value of different forests along the Escarpment to Wood Thrush population maintenance is important in developing suitable land-use plans within this landscape. Wood Thrush populations may be at risk if adjacent land-use management practices, such as logging and trail development, encourage increased parasitism rates in large and small forests along the Escarpment.

Methodology

Nests were located by exploring suitable habitat within known territories from May to August. Nest contents were monitored every 3 to 5 days with a mirror mounted on a telescopic pole. The status and condition of the nest was recorded including the number of host and cowbird eggs and young, estimated nestling age, as well as evidence of fledging, depredation or natural disturbance.

Attempts were made to band breeding males and females by capturing them with mist nests. Nestlings were banded when they were between 8 and 10 days old if they could be reached with a stepladder. Banding was a key component of this study to gather data on movements within and between nesting seasons.

Summary of Results

Significant variations in nesting success over the five year study period reflected the need for long term monitoring projects and not single year projects. 1998 remains the most successful year for Wood Thrush nesting success (62%).

When the data in each of the forest size classes were combined over the five years, nesting success in both small and large forests was similar. Optimistic survival estimates suggest that all of the studied forests, regardless of size, are potentially important breeding areas for Wood Thrush. What set large forests apart from small woodlots was the difference in parasitism rates; small woodlots consistently showed high rates of brood parasitism compared to large forests.

For more information, contact:

Lyle Friesen
Canadian Wildlife Service, Environment Canada
519-826-2092
Lyle.Friesen@ec.gc.ca