QUICK TIPS: Some bluebirds in some areas migrate, and others don't. We still don't know a whole lot about the "whys" of bluebird migration.
We still don't know a whole lot about bluebird migration patterns. In northern areas, and high-elevations, most bluebirds migrate, probably as a result of natural selection. The birds that did not migrate may have died due to long periods of cold temperatures and lack of food.
In southern areas, populations are mostly residents, with some "partial migrants". When these populations are hit by an extended period with subfreezing temperatures, with lots of snow and ice, they may not survive.
Survival can be affected by factors such as:
how long the severe weather lasts (e.g., Cliff Swallows will die if severe weather (very cold temperatures and rain) lasts four days or more [Brown, 1999.])
high thermal tolerance. Bluebirds can
conserve energy through "noctural hypothermia," lowering of body temperature by about 10-15 degrees to conserve energy on cold nights. Metabolism (e.g., heart rate and respiration) also slow down. Roosting also helps them use 13-28% less energy (Cooper, 1999, Kendleigh 1961)
the availability of shelter (i.e., roosting in nestboxes or cavities)communal roosting. (The most bluebirds recorded roosting in one cavity was 16 birds [Pitts, 1976.])
There is concern that suffocation can happen when large groups roost together in tight spaces, but I suspect hypothermia or starvation are more likely causes of death in these situations.
More info and References
BROWN CR AND CR BROWN. 1999. Natural selection on tail and bill morphology in Barn Swallows Hirundo rustica during severe weather. Ibis 151:652–659.
COOPER SJ. 1999. The thermal and energetic significance of cavity roosting in Mountain Chickadees and Juniper Titmice. Condor 101:863–866.
KENDEIGH SC. 1961. Energy of birds conserved by roosting in cavities. Wilson Bulletin 73:140–147.
PITTS TD. 1976. Fall and winter roosting habits of Carolina Chickadees. Wilson Bulletin 88:603–610.
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