Bluebirds ALMOST ALWAYS nest in cavities like nestboxes, hollowed out trees, etc. Cavity nesting poses several advantages, including decreased exposure to weather (e.g. hypothermia) and predators (including cowbirds.)
Birds like American Robins that typically nest out in the open (what I call "open cup nesters") have evolved differently from cavity nesters. Incubation periods may be shorter, newborn nestlings are often feathered (vs., altricial), and they grow faster and fledge earlier than most cavity nesters. This increases their chances of survival in a less protected environment.
However, Keith Kridler notes that on the island of Bermuda, Eastern Bluebirds were originally found using old Robin nests as a base for their nests in open branches of trees and bushes. When nestboxes or cavities are not available, there are occasional reports of bluebirds nesting in hanging baskets (although these usually turn out to be Robin nests), Topsy Turvy Tomato plant containers, old cliff/barn swallow nests, ledges of porches, and metal pipes.
In Leesburg, Virginia, Betty and Bill Ebert have been battling House Sparrows. Initially, bluebirds successfully used their nestboxes. But then House Sparrows quickly took over. "Before the HOSP invaded, ur bluebirds could nest wherever and whenever. How things have changed in such a short time. This year (2009), two pairs of bluebirds were forced to make other choices. "For over a month there was a battle over the box between bluebird couple and a house sparrow couple," reported Betty.We and the bluebirds gave up, took the nest box down. Soon after we discovered the bluebird nest in our deck raftersOne nested in the rafters under the Ebert's deck. To prevent House Sparrow harrassment, the Eberts hung mylar ribbons nearby. Five nestlings fledged successfully on July 13, 2009. The second pair nested in a Kinsman swallow nest. Four young fledged successfully on June 25, 2009.
Note: The Terracotta pot is sold by Kinsman, www.kinsmangarden.com. They also sell cement birdhouses by Schwegler, which most bluebird landlords do not recommend. I tried one out in 2009, and not even House Wrens used it.
You cannot begin to preserve any species of animal unless you preserve the habitat in which it dwells. Disturb or destroy that habitat and you will exterminate the species as surely as if you had shot it. So conservation means that you have to preserve forest and grassland, river and lake, even the sea itself. This is vital not only for the preservation of animal life generally, but for the future existence of man himself—a point that seems to escape many people.
-Gerald Durrell, The Nature Conservancy
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