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Frequently Asked Questions

1)  Do bird blow flies harm nestlings?
 In Netscape, Right-click and select View Image to see the full size GIF

The answer is, it depends.  Low to moderate populations seem to have little effect.  Nestling birds are real efficient blood generators and normally can replace blood as fast as larvae can remove it.  My studies showed that, when larval populations exceeded about 10 actively feeding 3rd instar larvae per nestling, nestlings became anemic and more susceptible to starvation, hypothermia and other parasites like mites, fleas, and lice. (Whitworth and Bennett 1992)

This means a nest with 4 nestlings would have to have 40 or more larvae.  In many areas, this would be an unusual condition, in some areas it is common.  If you would like a copy of a paper I published on this subject E-mail me with your address.

2) Should I check my nests for blow flies while nestlings are in it?

For the average birder, it's probably not necessary unless you have evidence of heavy infestations.  Even in heavily infested nests, nestlings are rarely killed by blow fly larvae.  Disturbing nestlings and parents by looking through their nest for larvae will cost birds energy, which may equal the energy used to replace blood lost to larvae.  Also, you are disturbing some nests that are not infested. When you disturb older nestlings, they may fledge prematurely, before they can fly well enough to survive.

If you suspect blow fly populations may be high in your area, inspect nestling abdomens for tiny scabs left by feeding larvae.  In heavy infestations you also may observe larvae attached to nestlings.  If you find heavily infested nests, you can physically remove larvae or do nest replacement.

If you want help evaluating your nests for blow flies, wait until nestlings fledge and send me your nests. Please include a copy of the form found under collecting bird nests.  I have a Federal U. S. Fish and Wildlife Permit which allows me to receive bird nests from homeowners.  I will tell you if the nests are infested and what the numbers are. 

3) Is there any reason not to control bird blow flies in nests?

Bird blow flies, like mites, fleas, diseases, weather and other adversity allow survival of the fittest to function and helps ensure that only the fittest nestlings survive to reproduce.  Nestlings which are protected from adversity may produce offspring that are less fit and less able to survive without human intervention.  Remember bird blow flies are perfectly natural and have co-evolved with birds for millions of years.  If they kill their hosts, they will die too.

This is not the final word on these issues, feel free to send me your comments. twhitworth@birdblowfly.com

Thanks to all who have sent me nests.


Terry Whitworth, Ph.D.
2533 Inter Avenue
Puyallup, WA 98372
Phone 253-845-1818
email: twhitworth@birdblowfly.com


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all content copyright © 2012 by Terry L Whitworth except as noted.

Portions of images by Joseph Berger (blowfly in header), John Triana (wren nest in title background), and Whitney Cranshaw (fly on wall), used by permission of www.insectimages.org.