BAM developed estimates of avian densities (males per hectare) for 70 species of boreal forest songbirds. These estimates were based on off-road survey data only; BBS route data were not included. The results are summarised by species under RESULTS.
Our earlier methodological research provides the foundation for these estimates that enables us to adjust the survey counts for the many, substantial, observational biases inherent in the BAM dataset (see Avian Data Analysis). Biases occur because of incomplete detection rates of birds during surveys, variability in detection rates relative to habitats and the times of day and year that surveys occur, variability in the survey protocols used by different studies, and uneven sampling of habitats and geographic strata.
We developed a hierarchical model that accounts for these sources of observation bias. Our models implemented many of the recommendations made to improve Partners in Flight's (PIF) estimates of landbird population size in North America by better accounting for sampling effects related to avian detection rates and uneven geographic and habitat coverage of surveys.
The model estimates breeding densities and population sizes of 70 species of songbirds as functions of geographic strata and habitat classes, and variation in habitat use among geographic units.
The hierarchical model that we used to estimate avian densities included the following specifications:
Estimating avian density and population is a complicated task. No single, definitive approach exists to calculate avian densities. Our density values, like all estimates, include an inherent level of uncertainty. They were calculated using one possible approach to correcting for the sampling issues that influence bird counts, based on these assumptions: