Disappearing Purple Martins

We regularly have purple martins in our yard during nesting season, but one year they were acting strange. A pair began building its nest, and then a few days later, disappeared. Do you know why they had a sudden change of plans?

—La Vonne Propst, Brownsville, Oregon

George: While it's disappointing to see the birds leave, there are several reasons that could have made them fly the coop. Because purple martins nest in colonies, and there was only one pair in your house, they may have left to move into a colony nearby. Another possibility is that they may have been chased away by European starlings or house sparrows, both nonnative pests that compete with native species. Or perhaps a cold snap, even a short one, limited the number of insects they could find around the birdhouse to keep them alive. It sounds like you have the right setup to attract purple martins to your backyard. I hope the success you've had from previous years returns next spring.