Shoreline development influences common loon territory selection but not extent of nest defense

Shoreline development of temperate lakes has increased steadily in recent decades, putting pressure on animal populations that live in lacustrine habitats. Common loons (Gavia immer) nest along lakeshores, and shoreline development has been identified as a likely cause of population decline in this species. Taking advantage of a marked loon population from a long-term study in northern Wisconsin, USA between 1992 and 2023, we investigated impacts of shoreline development and human disturbance on territory choice and nest defense behavior of breeding loons. Loons settled on territories that resembled their natal lakes in terms of shoreline development, an indication of natal habitat imprinting. Shoreline development did not affect the distance at which loons flushed from their nest at the approach of humans. However, nesting loons were less likely to flush from the nest late in the incubation period, suggesting a tendency for loons to defend nests according to likely fitness benefits. Loons' tendency to breed on lakes similar in human activity to their natal lake might reflect their effort to seek out lakes of familiar fish species composition rather than human activity. Natal habitat imprinting of this kind illustrates the potential of anthropogenic factors to affect behavior and, in turn, population dynamics.