New paper on detectability of boreal birds during roadside and forested point counts

Read about Ph.D. Candidate Dan Yip’s recently published work below, and find the full article here

Sampling design schematic for our experiments near Calling Lake, Alberta, Canada, July 22–August 24, 2014. Wildlife Acoustics SM2 recorders were placed on the forest edge (1) and on the road (2), and playbacks were conducted to both recorders simultaneously along a transect following the forest edge. For forest playbacks, recorders were placed within the interior forest (3) and playback transects ran perpendicular to the road. 2014.

Abstract:

Point counts are one of the most common ways of collecting data to determine the relative abundance of birds. Many studies and monitoring programs, including the North American Breeding Bird Survey, use relative differences in counts of birds to assess changes in abundance over time and space. Many factors influence whether relative differences in counts of birds between various environmental conditions are reflective of actual differences in bird density. A major assumption of relative abundance is that birds with different song frequencies and amplitudes are heard at the same distances in different environmental conditions. We compared sound transmission in forest habitats and along low-use forestry roads, and calculated detection radius for different species to test the assumption that differences in bird counts between forest interior and roadside locations reflect actual differences in bird abundance. A playback–recording experiment was used to broadcast sounds through forest interior, along a forest edge, and down forestry roads in conifer and deciduous forests to determine whether sound propagation differed across environments. Sound attenuated significantly faster in forests than along roads or forest edges. Similarly, the distance at which bird songs could be detected was significantly shorter in forest than along the road or forest edge for 20 of 25 species. We found the area surveyed to be up to twice as large on road compared to within forests, which suggests that roadside surveys might inflate avian density estimates in comparison to off-road counts. Local atmospheric conditions also influenced detection probability, but the magnitude of the effect was weaker than the land-cover effect. Major differences in detection between roads and interior forest suggest that comparisons of surveys conducted along roadsides and in forest areas should be done carefully if the goal is to make direct comparisons of abundance.

Influence of minimum frequency on effective detection distance (EDR, with 95% confidence intervals) for each (A) species and (B) tone along different transects (edge, forest, and road) near Calling Lake, Alberta, Canada, July 22– August 24, 2014. ​

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