Tree Swallow eggs

• Color: pure white, unmarked. There are no markings on a fresh egg, though eggs pick up stains from the nest as incubation goes on.
• Size: 17–20 mm long by 13–15 mm wide. For scale, a US quarter is 24.3 mm across, so this egg is roughly the width of a US quarter.
• Where you find it: nest box or tree cavity lined with feathers. Tree swallows also use nest boxes with a 1 1/2 in entrance, so eggs in a box are a strong clue.
• Clutch: 4–7 eggs. A nest with far more eggs than this may hold eggs from two females, or a cowbird egg laid by a brood parasite.

These species lay eggs in the same color group, sorted by how close they are in length. Size and nest site usually settle it.

Laying starts earliest in Arizona (Apr) and latest in Alaska (Jun). Months are estimates from regional breeding data shifted for latitude.

Eggs hatch after 11–20 days and the young leave 15–25 days later. See the full baby Tree Swallow guide for nestling stages and what to do if you find one on the ground.

• Leave them. Parents often step away for long stretches, especially before the full clutch is laid, because most songbirds do not start incubating until the last or second-to-last egg.
• A single egg on the ground is usually lost for good. Eggs cannot be hatched at home, and possession of native bird eggs is illegal without a permit.
• If a nest is somewhere inconvenient, wait it out. Most small songbirds are done in three to five weeks.
• Watch from a distance and keep pets indoors. Repeated visits can lead predators to the nest.