Purple Martins begin nest building around four-six weeks after they arrive at their final breeding site.
Purple Martins will typically build their nests out of straw, twigs, and pine needles. They may also build a mud dam in the front of the nest. Nest building can take three-four weeks to complete and both sexes will construct the nest.
The last stage in nest building is lining the bowl of the nest with green leaves. The function of the green leaves is still unknown; they may act as an insecticide, help regulate the temperature and moisture levels in the nest, or to help keep the nest clean. Purple Martins will normally start egg-laying soon after bringing in green leaves. The nest itself is flat, only about one to three inches high.
The female Purple Martin lays one egg per day, generally in the morning, for a total of two to eight pure white eggs. The average number of eggs per nest is four to six.
Incubation begins with the penultimate (second to last) egg laid. Only the female can incubate the eggs because they have a brood patch, a featherless area rich in blood vessels that transfers heat to the eggs. Males may sit on the eggs for short periods of time, but they cannot incubate the eggs.
Incubation lasts 15-16 days, although incubation can be delayed due to weather. Hatching may be spread out over two or three days. Click here to see the growth of the Purple Martin nestlings.
The young will fledge between 26-32 days after hatching and can receive care from both parents between one to two weeks after fledging.