Executive Summary.-The Boreal Avian Modelling (BAM) team examined avian survey point count data collected from across the boreal forest region of Canada to quantify roadside bias in both survey coverage and survey counts by the North American Breeding Bird Survey (BBS), the most important monitoring survey for a wide suite of bird species in the boreal. We compared data from the BBS to data from off-road surveys compiled by the Boreal Avian Modelling Project (BAM) to (1) determine whether gaps in survey coverage by the BBS might be covered by off-road surveys in the region, (2) test for roadside bias in survey counts by comparing data between roadside and off-road surveys, and (3) determine whether the statistical power to detect trends was similar between roadside and off-road surveys given comparable survey effort.• Survey coverage by the BBS was strongly biased toward sampling southern ecozones which make up 49% of the boreal but received 89% of the BBS surveys. Thus, the ecozones in the northern half of the boreal collectively comprise one of the largest gaps in survey coverage in North America; poorly sampled habitats across the boreal included conifer forests and woodlands, tundra-and lichen-dominated areas, wetlands, and areas with recent burns.• The patterns of proportional survey coverage by the BBS, in terms of geographic and habitat strata, mirrored the distribution of the road network and the sampling of off-road areas by BAM.Thus the BBS appears to (1) sample geographic areas and habitats in proportion to their availability along the road network and (2) draw from the same sampling frame as the off-road surveys included in BAM (i.e., the road network).• Opportunities exist to address some of the geographic sampling gaps in the northern boreal in areas serviced by existing roads. We identified 380 areas in the northern boreal with ≥30 km of road network, but with no prior surveys. These areas should be evaluated for future roadside surveys aimed at closing the large continental gap in avian survey coverage. However, complete coverage of sampling gaps will likely require a coordinated, national approach to off-road surveys in the northern boreal.
2• We found significant differences in survey counts for 79% of 85 species between roadside and off-road surveys (roadside bias). Positive roadside bias was more than twice the magnitude of negative roadside bias both in terms of number of species and average effect size. These results are similar to those from small-scale studies in the temperate zone. Extrapolating roadside results to off-road areas may overestimate population sizes for those species that exhibit positive roadside biases.• Although we controlled for habitat type at the point count location, our results indicated that (1) species showing positive roadside bias are often associated with agricultural areas, clearings, shrub habitats, habitat edges, or human structures and (2) species showing negative bias tend to be associated with mature forests or forest interiors. Controlling for additiona...