Population: Bluebirds need enough food to feed themselves and their families. If there are too many bluebirds in one area, the competition may be too intense, and they may move elsewhere.
Past nesting success: A bird that nested successfully is more likely to come back to that nestbox or area. In particular, House Sparrows and House Wrens can interfere with nesting success, and drive bluebirds from an area. Responsible nestbox monitoring and use of predator guards can increase the odds of success.
Spreading out: To avoid mating with family members, young often disperse from an area.
Nest sites: Nestboxes may attract bluebirds. Leaving nestboxes up year round MIGHT increase the likelihood that they will notice them in the fall (during migration) or roost in them during the winter, and thus stay in the area when breeding season begins.
Some studies show that about 11-15% of fledged Eastern Bluebirds come back to the area where they were born (Fiedler 1974, Plissner and Gowaty 1996).
Out of the birds that do return, one study showed that <1% bred in the box where they were born, and <2% bred near the box where they were born.
In some studies, 26% - 44% of adults come back to breed at the site where they nested the previous year. (Fielder 1974, Gowaty and Plissner 1997)
If the first nesting attempt fails, 69% of females may move. If their first attempt succeeds, 39% may still move to another location. (BNA).
References and More Information:
Gowaty, Patricia Adair and Jonathan H. Plissner. 1998. Eastern Bluebird (Sialia sialis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/381 doi:10.2173/bna.381
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