Birdhouse Guide

On freezing nights, cavity-nesters cluster for warmth. Pygmy Nuthatches in the western US form some of the largest known communal roosts of any North American songbird, with dozens of birds packed into a single cavity. Blue Tits and Great Tits across northern Europe roost in nest boxes through the coldest stretches of winter — communal roosting in tit-family species is extensively documented by the British Trust for Ornithology.

Sydeman et al., The Condor (1988); BTO winter roost surveys.

Cavity interiors stay measurably warmer than ambient air on cold nights. Black-capped Chickadees survive overnight at a far lower energetic cost when roosting in a cavity than in foliage — a difference that becomes the difference between life and death below −20 °C. Resident wrens, screech-owls, and bluebirds are routinely observed entering boxes during heavy rain and high wind.

Brittingham & Temple, The Auk (1988); Cornell Lab of Ornithology, NestWatch.

In replicated population-ecology experiments, adding nest boxes to cavity-poor sites lifts breeding density three- to ten-fold. Cavity availability — not predation, not food — is the binding constraint on cavity-nester populations across both North American and European study systems.

Newton, Population Limitation in Birds (Academic Press, 1998); Lambrechts et al., Acta Ornithologica (2010).