Four nesting pairs in a 12-room house is good occupancy. Extra rooms are used for roosting and for young, preflight birds. Also, sparrows will likely fill a few vacancies. Before the martins are nesting, leave the sparrow nests alone to avoid disturbing the martins. Martins usually lay four or five eggs.
Incubation is about two weeks, and age to first flight is about four weeks.
Martins build only one nest per year, but they may renest if the first attempt is an early failure. After flight, martins will return to the house for about two weeks to roost and perch. Martins feed on the wing and eat a variety of flying insects, including flies, beetles, moths and cicadas. They will not decimate bee hives. After aggregating in large flocks in late summer, they depart for wintering grounds in South America during late August and early September.