Factors driving territory size and breeding success in a threatened migratory songbird, the Canada Warbler

Successful conservation of migratory birds demands we understand how habitat factors on the breeding grounds influences breeding success. Multiple factors are known to directly influence breeding success in territorial songbirds. For example, greater food availability and fewer predators can have direct effects on breeding success. However, many of these same habitat factors can also result in higher conspecific density that may ultimately reduce breeding success through density dependence. In this case, there is a negative indirect effect of habitat on breeding success through its effects on conspecific density and territory size. Therefore, a key uncertainty facing land managers is whether important habitat attributes directly influence breeding success or indirectly influence breeding success through territory size. We used radio-telemetry, point-counts, vegetation sampling, predator observations, and insect sampling over two years to provide data on habitat selection of a steeply declining songbird species, the Canada Warbler (Cardellina canadensis). These data were then applied in a hierarchical path modeling framework and an AIC model selection approach to determine the habitat attributes that best predict breeding success. Canada Warblers had smaller territories in areas with high shrub cover, in the presence of red squirrels (Tamiasciurus hudsonicus), at shoreline sites relative to forest-interior sites and as conspecific density increased. Breeding success was lower for birds with smaller territories, which suggests competition for limited food resources, but there was no direct evidence that food availability influenced territory size or breeding success. The negative relationship between shrub cover and territory size in our study may arise because these specific habitat conditions are spatially heterogeneous, whereby individuals pack into patches of preferred breeding habitat scattered throughout the landscape, resulting in reduced territory size and an associated reduction in resource availability per territory. Our results therefore highlight the importance of considering direct and indirect effects for Canada warblers; efforts to increase the amount of breeding habitat may ultimately result in lower breeding success if habitat availability is limited and negative density dependent effects occur.

Mortality of Migratory Birds from Marine Commercial Fisheries and Offshore Oil and Gas Production in Canada

There is an imminent need for conservation and best-practice management efforts in marine ecosystems where global-scale declines in the biodiversity and biomass of large vertebrate predators are increasing and marine communities are being altered. We examine two marine-based industries that incidentally take migratory birds in Canada: (1) commercial fisheries, through bycatch, and (2) offshore oil and gas exploration, development, and production. We summarize information from the scientific literature and technical reports and also present new information from recently analyzed data to assess the magnitude and scope of mortality. Fisheries bycatch was responsible for the highest levels of incidental take of migratory bird species; estimated combined take in the longline, gillnet, and bottom otter trawl fisheries within the Atlantic, including the Gulf of St. Lawrence, and Pacific regions was 2679 to 45,586 birds per year. For the offshore oil and gas sector, mortality estimates ranged from 188 to 4494 deaths per year due to the discharge of produced waters resulting in oil sheens and collisions with platforms and vessels; however these estimates for the oil and gas sector are based on many untested assumptions. In spite of the uncertainties, we feel levels of mortality from these two industries are unlikely to affect the marine bird community in Canada, but some effects on local populations from bycatch are likely. Further research and monitoring will be required to: (1) better estimate fisheries-related mortality for vulnerable species and populations that may be impacted by local fisheries, (2) determine the effects of oil sheens from produced waters, and attraction to platforms and associated mortality from collisions, sheens, and flaring, so that better estimates of mortality from the offshore oil and gas sector can be obtained, and (3) determine impacts associated with accidental spills, which are not included in our current assessment. With a better understanding of the direct mortality of marine birds from industry, appropriate mitigation and management actions can be implemented. Cooperation from industry for data collection, research to fill knowledge gaps, and implementation of mitigation approaches will all be needed to conserve marine birds in Canada.