Assessing Black Tern (Chlidonias niger) Habitat Associations in Saskatchewan, Canada, Using Aerial Imagery

Abstract. Wetland degradation throughout the interior of North America has resulted in a loss of breeding habitat for many waterbird species. The Black Tern (Chlidonias niger) is an obligate marsh-breeding colonial waterbird that has experienced widespread, long-term population declines. Habitat loss and degradation through agricultural conversion, wetland drainage, and agrochemical runoff have been identified as key threats, and studies have suggested that a decline in breeding habitat may be a contributing factor to population declines. Habitat association studies have noted relationships between Black Terns and wetland characteristics, including both local-scale factors such as vegetation type, and landscape-scale factors such as wetland density. However, similar studies have not been conducted in Saskatchewan, the core of the species range in North America. We used high-resolution remotely-sensed imagery to relate habitat, land use, and geographic covariates at wetlands in Saskatchewan to the occurrence of breeding Black Terns and numbers at their colonies. We found that colony occurrence was positively associated with the extent of emergent aquatic vegetation present at a wetland. There was a strong non-linear effect of latitude, whereby colony occurrence and abundance were highest at mid-latitudes in Saskatchewan, corresponding to the boreal transition zone between the prairies to the south and boreal forest to the north. Our results suggest that Black Terns may be first selecting habitat at the landscape scale, perhaps in relation to wetland density, then occupying specific breeding colonies based on wetland characteristics.