Male Lincoln’s Sparrows arrive to the breeding ground in mid or late May and begin to sing in order to attract a mate. In early June, females build their nests on the ground under dense grass or shrub cover, usually inside a low willow shrub, mountain birch, or sunken in a depression of sphagnum moss Their nest is a well-covered shallow open cup of grasses or sedges. Clutch size is typically 3-5 eggs which are oval in shape and colored pale green to greenish-white and spotted reddish brown. One egg is laid per day, and females begin incubating eggs before the clutch is complete, while males do not incubate. Incubation lasts for about 12-14 days. Young are born altricial and leave the nest about 9-12 days after hatching, although they may be cared for by their parents for another 2-3 weeks. Fledglings are mostly flightless their first day, but their flying abilities quickly improve, and by day six they can fly more than 10 meters at a time.
Song Sparrows’ nests are parasitized by the brown-headed cowbird. The cowbirds’ eggs closely resemble song sparrows’ eggs, although the cowbirds’ eggs are slightly larger. Song sparrows recognize cowbirds as a threat and attack the cowbirds when they are near the nest. There is some evidence that this behavior is learned rather than instinctual. A more recent study found that the behavior of attacking female cowbirds near nests may actually attract cowbird parasitism because the female cowbirds use such behavior to identify female song sparrows that are more likely to successfully raise a cowbird chick. One study found that while cowbird parasitism did result in more nest failure, overall there were negligible effects on song sparrow populations when cowbirds were introduced to an island. The study pointed to a number of explanatory factors including song sparrows raising multiple broods, and song sparrows’ abilities to raise cowbird chicks with their own.