Because of the starlings' high fertility as well as its polygyny, and its ability to utalize a broad spectrum of foods and habitats starlings are able to both multiply and invade rapidly. Because of their increasing numbers and aggressive behavior, they are outcompeting native birds in North America which have historically utilized cavities as nest sites. This has resulted in population declines for various wren, swallow, and bluebird populations. Starlings are very gregarious birds, flocking at all times with other starlings. These birds breed in bunches, feed in flocks, and migrate in masses.
Males also impress their females by singing in their nests and by decorating their nests.
When the female is fertile, her male mate closely surveys his competitors. The male follows the female in every activity she takes part in. Another notable aspect of starling behavior is its relationship with humans. In addition to being very gregarious, starlings are also amenable to human disturbance, coexisting with people in urban areas. The European Starling is a bird of lowlands, found mainly on non-mountainous terrain.