This may be because they can't decide which box to use (see indecision), or it may be to deter other birds from nesting in the second box. Sometimes the second nest is used for a second brood. Problems and Predators : Sometimes a nest is started and then abandoned because of a problem.

Why do House wrens abandon their nests?

Monitoring: May abandon nests during construction or egg laying if disturbed (or female is captured). Some show no fear of humans when nesting in outbuildings. Adult may chatter loudly when nest area is approached.

Why would Wren abandoned nest with eggs?

During breeding season, wrens who are injured or killed may leave behind a nest full of eggs. ... Incubating eggs and caring for hatchlings is challenging, so if you find an abandoned nest, the best option is contacting a trained wildlife rehabilitation expert.