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Northern Prairie Wildlife Research Center

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Eastern Bluebird
(Sialia sialis)

Exotic Species Control

A major factor leading to the decline of eastern bluebird populations has been nesting competition with non-native species, specifically European starlings and house sparrows. Simple steps can be taken, however, to prevent the use of boxes by exotic species. By using the nest box entrance hole dimensions shown in figure 1, starlings should be prevented from entering nest boxes. However, house sparrows will continue to be a problem. Sparrows will aggressively compete with native species for nesting sites, going so far as to destroy the eggs and kill the nestlings of native cavity-nesting birds. Boxes should be constructed without a perch post to discourage sparrow use. If house sparrows are found to be nesting in a box, remove and dispose of the nest and its contents. Trapping and eliminating the adult bird in the box is more effective than simply removing the nests. Trapping with inside-the-box traps or bait-type traps such as those made by Trio, and Hav-a-hart, is recommended. If uncomfortable with removing the birds or nests, another option is to vigorously shake the eggs, or addle them, for 60 seconds and leave them in the nest so that the female will continue to expend reproductive energy without hatching success. This also prevents the exotic species from attempting to take over another box. Other native cavity-nesting birds, such as Carolina and black-capped chickadee, tree swallow, house wren, house finch, tufted titmouse, and great-crested flycatcher may use and are welcomed in nest boxes. Because they are federally protected, it is illegal to destroy them, their nests, or their eggs. Therefore, nest monitors must be certain of the bird species nesting in a box before control measures are implemented. Two aids, Guide to Nests, Eggs, and Nestlings of North American Birds, and Birds' Nests, in the Peterson Field Guide Series are recommended (complete citings included in the reference section). Note: Box placement is the best method to control exotic species. Careful box placement will also reduce competition from tree swallows and house wrens. Placing boxes a good distance from buildings and human habitation will work to prevent house sparrows from using bluebird boxes. Using box pairing and placing boxes in open grassy areas away from brushy, woody vegetation will help to reduce over-use of boxes by non-target natives such as tree swallows and house wrens.
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