|dc.description.abstract||Black Tern (Chlidonias niger) populations are declining across their range in North America, but it is unknown at what point during the annual cycle populations may be limited. Although causes of decline remain largely unknown, past studies showed that conditions on stop-over and over-wintering locations may be important factors. Little is known about the migration timing, routes, stop-over sites, or the range and environmental conditions of over-wintering Black Terns. Furthermore, it is unclear what habitat qualities and attributes influence Black Tern colony occurrence in the core of their breeding range in the Canadian prairies. Breeding habitat selection is increasingly important in the face of continuing and potentially damaging anthropogenic land uses and climate change. My objectives were to 1) examine the influence of various habitat, land use, and geographic covariates on Black Tern occupancy and abundance in the core of the species range in Saskatchewan; and 2) examine full annual cycle movements and potential differences in migratory stop-over and over-wintering locations from Black Terns breeding across their range in central (Saskatchewan, Canada) and eastern (Ontario, Canada and Michigan, USA) North America, while providing a preliminary assessment of migratory connectivity.
I assessed Black Tern occupancy and abundance at 68 wetlands compiled from historic waterbird databases and field surveys. I used aerial imagery to quantify four habitat covariates I predicted would influence Black Tern site suitability: total wetland area, the extent of emergent vegetation present at each wetland, wetland classification, and the degree of anthropogenic encroachment surrounding each wetland. I deployed 88 archival light-level geolocators on Black Terns across five colonies in Saskatchewan and Ontario, Canada, and Michigan, USA, during the breeding seasons of 2016-2019 to track their migratory movements, stop-over, and over-wintering locations.
Habitat association results showed that colony occurrence was predicted by both geographic and habitat covariates, with wetlands at mid-latitudes and with more emergent vegetation associated with an increased likelihood of Black Tern colony occurrence. Abundance was best predicted by geographic patterns (latitude) alone. These results revealed that Black Terns are selecting breeding habitat at multiple scales primarily along a narrow latitudinal band across Saskatchewan. This suggests that individuals may first be locating breeding sites based on large-scale landscape features driven by latitude, and then selecting specific wetland locations for breeding based on small-scale features such as emergent aquatic vegetation. The highest abundances corresponded with the boreal transition zone between the prairies to the south and boreal forest to the north, an area that might be preferred by Black Terns due to its abundance of wetland habitat.
The migratory routes used by Black Terns varied among individuals, but some geographic patterns emerged; Black Terns from central and eastern regions used different migratory routes. All tagged individuals spent some or most of the wintering period at or off the coast of Panama, highlighting the importance of this location during the wintering period. These results suggest Panama might be a key location of population mixing on the wintering grounds. Additionally, migratory connectivity was stronger between breeding sites and the first migratory stopover location than it was between breeding sites and wintering locations, indicating that individuals from different breeding colonies generally remained more closely linked to other individuals from their same breeding colony during the fall migration, but that subsequent mixing of the populations occurred among individuals from multiple breeding locations on the wintering grounds. Some interesting patterns of Black Tern breeding dispersal were also uncovered based on long-distance breeding region switching of one individual tracked over multiple years. Furthermore, preliminary insights into colony abandonment and short-distance breeding dispersal of Black Terns in Saskatchewan were also revealed.
These results fill important data gaps that exist for the North American Black Tern. Although Black Tern populations are declining across their range, I demonstrate that colony occurrence and abundance can be predicted using both geographic and habitat models. These associations have revealed evidence that individuals are selecting habitat at multiple scales, both large and small. The quadratic relationship between the likelihood of colony occurrence and the latitude of a wetland suggests that large-scale landscape variables, such as variation in wetland density, may be driving this pattern. Very little was previously known about the Black Tern full annual cycle, and these migratory tracks are the first that have been collected for the species. Despite the use of different flyways across the breeding range, tracked individuals showed a high degree of mixing and weak migratory connectivity on the wintering grounds. A concentration of birds in Panama suggests that localized threats in this area could have an impact across the breeding range. In addition, evidence of long- and short-distance breeding dispersal suggest a lack of population structure and high levels of movement and gene flow. My results provide some of the first detailed migratory tracks for the Black Tern in North America, and reveal important habitat associations in the core of the species range, critical information that could benefit the conservation of the species.||