You return from vacation to find a stranger living in your home, a stranger who refuses to leave. You go to a neighbor's house for help, and strangers are living there, too. Sound like a scene from the Twilight Zone? Well, it's what happens sometimes to native animals when new species arrive from elsewhere. Consider the trials faced by the Eastern Bluebird.
Back in 1890 a few well-intentioned bird-lovers decided to release a hundred European Starlings in New York's Central Park. The birds adapted easily to their surroundings and soon spread to cities across the Northeast. As a consequence, though, many native Eastern Bluebirds lost their homes.
The reason? Both species nest in holes -- tree cavities and the like -- but only bluebirds migrate for the winter. So while the bluebirds were sunning themselves down south, the recently arrived starlings moved into the vacated homes. And since bluebirds are gentle and nonconfrontational by nature, they had no choice when they returned north but to look for new holes in which to build their nests. But even when they did find suitable replacements, chances were good that the thuggish starlings would soon evict them from these, too.
Over time bluebird numbers dropped to the point where conservationists feared the birds would disappear completely. Realizing this, the same creatures responsible for the mess -- humans, that is -- decided to find a solution. It's simple but effective: nest boxes with holes perfect for bluebirds but too small for starlings. And with tens of thousands of these boxes now gracing backyards in America and Canada, the Eastern Bluebird appears to be on the rebound.