Analyses to support critical habitat identification for Canada Warbler, Olive-sided Flycatcher, and Common Nighthawk: Final Report 1 and 2

Report 1 Overview: Defining critical habitat is required to inform recovery strategies for species at risk. High density areas tend to have the highest per unit area reproductive output. Hence, these areas are often assumed to be the most important to protect if populations are to recover. In this study, the Boreal Avian Modelling (BAM) project provided analyses to support Environment Canada in its effort to identify critical habitat for three Neotropical migratory birds (Canada Warbler, Cardellina canadensis; Olive-sided Flycatcher, Contopus cooperi; and Common Nighthawk, Chordeiles minor). Specifically, BAM used the best information available (i.e. avian point count and biophysical data) to: 1) generate habitat models identifying the biophysical attributes characterizing areas of low and high densities; 2) estimate population size at multiple spatial scales under various assumptions; 3) map predicted density estimates and uncertainty across Canada; and 4) provide a Schedule of Studies identifying important gaps in data availability and the limitations of current models.

Information from over 1.5 million avian point count surveys were used with land cover, disturbance, topography, climate, and spatio-temporal variables to generate 9 model subsets that explained the variation in abundance of the three focal species across Canada. We used different model subsets to account for different spatial extents and co-linearity among variables. Poisson log-linear regression models were produced using a branching hierarchy model building process and bootstrap procedures to account for model uncertainty.

Density of male Canada Warblers was higher eastern Canada and in mixedwood and deciduous stands with tall trees and closed canopy. There was a 50-60% decline in relative abundance of territorial males from 1997 to 2013. Density was lower in areas with high proportion of agricultural and human developments within a 16 km2 area of survey points. Alternatively, landscapes with a higher proportion of mixedwood and deciduous stands supported higher densities. Estimated Canadian population size across the monitoring period was 11 million birds with the highest proportion being in Ontario, Quebec, and Alberta.

Density of male Olive-sided Flycatchers was higher in conifer stands, recent burns, shrubby areas, and western Canada. There tended to be higher a density in stands with taller trees. There was no significant evidence of a temporal trend in relative abundance of territorial males from 1997 to 2013. Landscapes (i.e. 16 km2) supporting higher densities included high proportions of conifer and mixedwood stands, shrubby and wet areas and water bodies. Estimated Canadian population size across the monitoring period was 9.2 million birds with the highest proportions of the Canadian population being in Quebec and British Columbia.

There was some evidence that shrubby areas, human development, and western Canada supported higher densities of male Common Nighthawks. Relative abundance declined by 70-80% from 1997 to 2013. Highly suitable landscapes were comprised of many land cover types (i.e. shrub and grassy areas, conifer stands, and, to a lesser extent, urban development, barren ground, and wet areas). The number of individuals in Canada was estimated at 270,000 with the highest proportions predicted to occur in Alberta, British Columbia, and Saskatchewan.

Model validation suggested that our habitat models had very good predictive and discriminatory power. This was determined based on agreement among bootstrap predictions and measures of goodness of fit for model subsets based on random samples of observation. We also compared our population estimates to those derived from data and methods used by Partners in Flight. This exercise highlighted important discrepancies in population size estimates when using different analytical approaches and including off-road surveys. Although our habitat models were derived from the best available information, we provided a detailed list of future studies required to address remaining gaps in the breeding ecology and habitat modelling of these species.

Report 2: Proposed Schedule of Studies identifying limitations regarding our modelling approach and knowledge gaps pertaining to the identification of critical habitat for Canada Warbler (Cardellina Canadensis; CAWA), Olive-sided Flycatcher (Contopus cooperi; OSFL), and Common Nighthawk (Chordeiles minor; CONI).