Post-fledging survival and habitat use of juvenile Least Bitterns (Ixobrychus exilis).

The post-fledging period, defined as the period after a young bird leaves its nest, is a crucial stage for birds and is often when they face the highest risk of mortality. Current management for breeding birds has been primarily informed by the nesting and adult stages while the post-fledging period is often understudied, despite birds spending an equal or greater amount of time in the post-fledging period than they do in the nest. In 2020 and 2021 we documented habitat used, including water depth and vegetation height, cover, and type, and survival of 50 fledglings of Least Bittern (Ixobrychus exilis), a cryptic marsh bird of conservation concern in the midwestern United States. We compared habitat used by fledglings to habitat at randomly selected points within a daily dispersal distance determined by the fledgling's age. Fledglings used points with denser and taller marsh vegetation than random points and they relied on persistent emergent vegetation, which likely provides greater predator protection, thermal refuge, and foraging opportunities. Fledgling habitat use was age-dependent and related to flight ability, with younger, pre-flight fledglings using denser habitats and older juveniles with flight ability using more open habitat. Additionally, 86% of Least Bittern fledglings survived until 30 d post-hatch (when they often left the marsh they hatched in), a higher survival rate than in the limited previous studies of fledglings. Mortality resulted from predation and exposure, and the greatest mortality risk was in the 3 d following fledging and in 2020. Our results suggest that managing for a diversity of marsh habitats, including both densely vegetated habitats commonly used by nesting Least Bitterns and more open marsh habitats, is important for fledglings. [ABSTRACT FROM AUTHOR]