All bluebirds are cavity-nesting species, and they need safe, secure locations to raise their broods. Unfortunately, they are not assertive, and more aggressive species can easily drive bluebirds out of prime nesting spaces. European starlings and house sparrows, both invasive species, will easily usurp nesting cavities, evicting and even injuring or killing bluebirds in the process. Bluebirds may also be subject to brood parasitism from brown-headed cowbirds, and young cowbird chicks can smother bluebird hatchlings and keep them from getting sufficient food and care. Continuing human development, particularly in the eastern bluebird's range, has removed many natural cavities these birds need for successful nests, making proper birdhouses even more critical.