Castle Rock, Colorado, USA
A nest box had been occupied first by a Western Bluebird pair and actually had the first confirmed eggs of the 2018 season. The first clutch had 5 eggs but only 3 WEBL chicks successfully fledged from the box as of June 3. When the box was checked one week later (June 10), a new nest was observed with one bluebird egg, one swallow egg and one additional swallow egg was on the ground along with an adult Tree Swallow seen in the area. Another bluebird egg was laid the next day but adult bluebirds were not seen near the box again. The nest remained as a Tree Swallow nest that fledged 3 chicks but the bluebird eggs never hatched.