Patterns of inter- and intraspecific nest dispersion in colonies of gulls and grebes based on drone imagery

Colonial nesting of both single and mixed-species assemblages is common across avian taxa. Coloniality may provide fitness advantages such as reduced nest predation through predator swamping, early warning of predators, or nest stability, or could simply arise through individuals selecting similar high-quality habitats. Traditionally, studies of nest dispersion in marsh-nesting colonial birds have required the use of on-the-ground methods due to the difficulty of accessing nesting areas and the cryptic nature of nests. Here, I make use of drone-borne thermal and visible sensors to capture imagery of a large sample of nests (~1,700) at single and mixed-species colonies of two grebe species and one gull species nesting at four marshes in Saskatchewan, Canada. As predicted, species exhibited significant intraspecific nest clustering, with individuals nesting closer together than expected by chance more often than not (in nine of 13 species-subcolony comparisons, where subcolonies were delineated based on aggregations of nests within marshes). In contrast, grebe species rarely nested closer to Franklin's Gulls (Leucophaeus pipixcan) than expected by chance, contrary to theoretical predictions based on presumed advantages of anti-predator defense or nest stability from gulls. As predicted, the larger (Western Grebe, Aechmophorus occidentalis) and more aggressive species (Franklin's Gull) had greater nearest neighbor distances than the smallest species (Eared Grebe, Podiceps nigricollis). Nearest neighbor distances tended to be smaller at subcolonies with greater emergent vegetation cover and at larger subcolonies, which may be due to reduced visibility allowing for smaller distances between neighbors, and social attraction at larger subcolonies, respectively. While valuable from a population monitoring perspective, this study demonstrates that high-resolution imagery from drones can also aid in answering ecological questions, and in combination with detailed nest success information, could help identify habitat needs and potential factors limiting reproductive success of declining and sensitive marsh species.