In winter, chickadees live in flocks of six to 10 birds with one dominant pair. A flock will defend its territory against other chickadee flocks. At night chickadees roost individually in tree cavities or among dense boughs of conifers. A roosting bird tucks its head under a wing to conserve body heat. On cold nights, a chickadee's temperature drops from a normal 108oF. to about 50oF., causing the bird to enter a state of regulated hypothermia, which saves significant amounts of energy.
Chickadees mate for life.
In spring, the winter flocks break up as pairs claim nesting territories ranging from three to 10 acres in size. Chickadees nest in May and June. The usual site is a hole in a tree, dug out by both sexes. Birch is a favorite, because this tree's tough outer bark stays intact after the inner wood rots and becomes soft enough for chickadees to excavate. Chickadees also clear out cavities in aspen, alder, willow, and cherry trees, and use abandoned woodpecker holes.