Roost characteristics and fidelity of silver-haired bat maternity colonies in a floodplain cottonwood forest

The maternal roosting behavior of migratory, tree-dwelling bat species remains poorly understood. From 2016 to 2020 we radio-tagged 53 adult female silver-haired bats (Lasionycteris noctivagans) and tracked them to their day roosts to evaluate roosting habits in a cottonwood (Populus) forest in the floodplain of the Bitterroot River in western Montana. We investigated roost tree characteristics of maternity colonies such as canopy closure, tree size, and decay stage, as well as the daily roost fidelity of tagged bats. Additionally, we marked 55 bats with passive integrated transponder (PIT) tags to assess interannual fidelity to summer habitat. We tracked bats to 94 unique roost trees. Based on previous studies, we expected silver-haired bat maternity colonies to roost in tall, large diameter trees. We modeled roosting probability as a function of tree height, day of year, and the interaction of these terms. We found bats preferred taller trees earlier in the year, but overall roosting probability decreased throughout the season and tree height had lesser influence on roosting probability later in the year. Bats used tall trees with high canopy closure in early stages of decay during periods of pregnancy and lactation, while the effect of these tree attributes lessened later in the season, perhaps after lactation when energetic constraints were reduced for adult females to prepare for migration by entering torpor during the day. Bats used an average of 5.2 roost trees during the young rearing period and remained in the same roost for an average of 1.5 consecutive days. While daily roost fidelity was low, we documented interannual fidelity to summer habitat from nine individuals. We found silver-haired bat roosting preferences vary with time, are tree-specific, and policies targeting conservation of the species should manage flood regimes to promote cottonwood establishment, height growth, and a mosaic of riparian habitats.