Martins typically lay one clutch, of four or five dull white eggs per summer.
Females do all the incubating, or sitting on the eggs to warm them until they hatch,
although males may also enter the nesting compartment during incubation.
The female spends 70 to 80 percent of the daylight hours incubating the eggs,
which hatch after 15 to 18 days. The hatchlings have no feathers and so the female
broods, or warms, them until they are seven or eight days old. During the first 12 days
in the nest, the young martins grow from about 3 grams at hatching to 40 to 45 grams.