Occupancy of tree cavities as they age: selection for young, mid-aged and older nesting cavities by cavity-nesting vertebrates

COCKLE, K.L.; WIEBE, KAREN; EDWORTHY, AMANDA B.; MARTIN, KATHY; TRZCINSKI, M. KURTIS

Lansing

Congreso; Joint meeting of the American Ornithological Society and Society of Canadian Ornithologists; 2017

Tree cavities are a critical nesting resource for many forest birds and mammals. In temperate systems, cavities can last for more than 25 years and their characteristics change with time, potentially influencing their value as nesting sites. In the context of wildlife and forest management, we investigate the relative value of generating a supply of fresh cavities?which are thought to be of high quality?versus protecting cavities as they age and expand in interior volume. For 20 years (1995-2015), we monitored the formation and occupancy of tree cavities used by more than 30 species of birds and mammals in interior British Columbia, Canada. Cavity occupancy by secondary users was highest 1-year post-excavation (66%), then declined to 37% after two years, remained at 33 ± 7% between 3 and 16 years of age, and increased to 50% use from 17?19 yrs post-excavation. Excavators that reused cavities (woodpeckers, nuthatches) strongly selected for one and two year old cavities, while large-bodied species (ducks, raptors, squirrels) selected mid-aged cavities and mountain bluebirds and tree swallows selected the oldest cavities. Cavities in living aspen trees, especially those excavated by northern flickers maintained occupancy by secondary users well across cavity age, and provided the bulk of cavities used in this system. Our results emphasize the need for diverse excavator communities to generate a supply of fresh cavities, but also the need to retain the mid-aged and older cavities to provide nest sites for large-bodied species (as cavities increase in volume with age).