Excavation or nest site selection: Generally migratory, may be resident. If not, arrives in early spring (February - March, depending on location), with nesting starting later in higher elevations. Males show off cavities, and the female makes the final choice. Good sign, but no guarantee, if female enters box during house hunting.
Nest construction: 1-7 days (typically 4-7), mainly in the morning. May start in early March to late April or early May depending on location. Nest of dry grass, weed stems, pine needles, twigs, straw, rootlets, shreds of dry bark, and, sometimes, wool, hair (e.g., deer or horse), or feathers. Deep (~2 inches), well-formed cup lined with finer materials, and occasionally a few feathers or trash (shredded paper, plastic wrappers.) Built by female (although male may bring in some nesting material, especially during courtship.)
Egg laying: Usually begins a day or two after nest is complete (depends on weather.) See typical first egg dates by State. Depending on climate, the first egg may be in late March to the first week of May. One egg is laid each day until clutch is complete - typically 5-6 for the first clutch, 4-5 for second, up to 8. Eggs are oval, smooth, glossy, unmarked, and are pale blue, bluish-white, or, rarely, white. Male sits near nest site to defend.
Incubation 13-15 days, depending on location and temperatures. Begins with the last or second to last (penultimate) egg. Only the female incubates (since only she has a brood patch) but the male may sit on the eggs when she is out feeding. The female sleeps in the cavity during incubation, while the male roosts nearby.
Hatching: Usually occurs over a 24 hour period. Asynchronous hatching is possible. Young are altricial (blind and naked). The female broods them intermittently for a week or longer. The female may prevent the male from feeding young while she is brooding (taking the food from him, and sometimes eating it herself.)
Development: From hatching to day 5, young are fed soft bodied insects and larva. After that, spiders and hard-bodied insects are fed. Parents remove fecal sacs throughout nesting. For photos and detailed description of nestling development, see Pearman's book.
Day 1: Eyes closed, pinkish skin with sparse down. Uncoordinated, raising head weakly and unsteadily, faint vocalizations.
Day 3: ear openings evident.
Day 5: feather tracts are all dark. Days 5-8, eyes open.
Day 7: feathers fully broken through all tracts except wings
Day 12: color on wings first visible
Day 13: Down feathers on crown stick out, no exposed skin except abdomen, may clacks. Avoid opening box after Day 13 to prevent premature fledging.
Fledging: 17-22 days +/- depending on weather and food availability. For the first three days, they are very dependent on parents, and by about 10 days begin capturing and preparing their prey. In 3-4 weeks they are on their own. Young get their adult plumage over the summer. If the female starts another brood, the male will care for the first brood.
Dispersal: Juveniles often remain with parents throughout summer and fall. After breeding, flocks may move to other feeding grounds, and then reappear in late summer or early fall. They leave their breeding grounds from August (Power) or September to November, depending on location. 76About 1 to 4 percent of banded nestlings return to within 15 miles of their natal grounds the following year (Pearman)
Number of broods: Last broods are usually out of the nest by August. If the first brood is successful, about half of pairs may attempt second brood if food supply and weather are good (Power). In the southern part of their range (and recently as far north as eastern Alberta, Canada), may raise three broods. Failures of second nestings are common (40% or more?). Unsuccessful females may change territories or nestbox types between clutches. (Power)
Longevity: Nine years is the record for banded birds. Ellis Bird Farm has recorded an 8 year old male and a 7 year old female (Pearman) See more info.