The tree swallow lays a clutch of two to eight, although usually four to seven, pure white, and translucent at laying, eggs that measure about . These eggs are incubated by the female, usually after the second-to-last egg is laid, for 11 to 20 days, although most hatch after 14 to 15 days. About 88% of nests produce at least one nestling, but this can be lowered by poor weather and a younger breeding female. The eggs generally hatch in the order they were laid. They also hatch slightly asynchronously, with an average of 28 hours between when the first and final nestling emerges. This can result in a weight hierarchy where earlier-hatched chicks weigh more (especially early in the nestling period) than those hatched later, allowing the female to prioritize which chick to give food to during food shortages. This likely has its greatest effect early in the nestling period, as by 12 days after hatching, there are generally no significant weight differences. Infanticide of the chicks and eggs sometimes occurs when a male is replaced by another male. Infanticide usually does not occur when the clutch is not complete, as replacement males then have a chance to fertilize at least one egg. When the male arrives during incubation, it sometimes commits infanticide, but other times adopts the eggs, as there is a chance that some eggs were sired from the replacement male. If the replacement male arrives after the chicks hatch, infanticide is usually committed, though the female will sometimes prevent this.