Mike Seamans wrote:

"I have long assumed that it was important to clean out my chickadee and
swallow nestboxes once a year. Does this mean that I should stop? Or
is that reading too much into these findings, applying them too broadly?
Perhaps their study was conducted in a much colder climate, where all
the mites were killed off in the winter. Are mites the impetus for
cleaning out nestboxes? Or is it something else? Thoughts???"

Jon. Anderson wrote:
Autumn 1996 issue (Vol 67 No 4) of the Journal of Field Ornithology,=20
Does Removal of Old Nests from Nestboxes by Researchers Affect Mite =
Populations in Subsequent Nests of House Wrens? Andrew J Pacejka and =
Charles F Thompson. pp 558-564. House Wrens prefer nestboxes that =
contain old nests to those where old nests have been removed, but remove =
(at least partially) the old nests themselves; Levels of mites are the =
same when researchers remove old nests and when they don't; inclusion of =
old nests in boxes may enhance the attractiveness of artificial nest =
sites.

Mike,

There is no consensus nor a quantitative basis to answer the question of =
whether old nests should be removed from nest boxes.

My reading of Pacejka and Thompson's paper was that House Wrens "clean" =
the old nests from their cavity nesting sites anyway, and that the level =
of parasitic mites (Dermanyssus hirundinis) and scavenging mites =
(Androlaelaps casalis) was not different in nests where the researchers =
removed the old nests or in unmanipulated nestboxes. When male House =
Wrens remove old nesting material, they likely remove many mites - =
presumably as many as did researchers when they removed nests from =
nestboxes.

The following information is plagarized liberally from their paper:

When active, fowl mites live about 10 days, and breed during the host's =
nesting period. They overwinter in old nests as adult females and eggs. =
The mite's life cycle consists of 5 stages: egg, larva, protonymph, =
deutomynph, and adult. With the exception of the egg and larval stages, =
at least one blood meal is necessary to develop from one stage to the =
next. Females require a blood meal before laying a clutch of about 20 =
eggs.

The authors note that species that reuse old nests usually have higher =
parasite loads than species that use a nest only once. Exposure to =
parasites in the nest may be detrimental to birds (such as =
threatened/endangered species that might have reduced genetic variation =
associated with small population sizes) that are more susceptible to =
parasitic infection. It is important to determine whether a species =
removes old nests from cavities and whether they are adversely affected =
by ectoparasites in the nests. If, as with House Wrens, the species is =
not usually affected by the parasites and they exhibit a preference for =
nestboxes that contain old nests, it would "behoove investigators" not =
to remove old nests, because leaving the old nests may enhance the =
attractiveness of artificial nest sites.

Two explanations for why boxes containing old nests are not avoided are =
that the effects of the mites are inconsequential and therefore the =
presence of mites doesn't play an important role in nest-site selection, =
or the detrimental effects of mites is offset by the benefits of a good =
nesting site (as indicated to the bird by the presence of an old =
nest...)

I imagine that the situation differs with different species and in =
different places. In southern Europe, researchers found that Pied =
Flycatchers prefer to nest in *empty* nestboxes because fleas decrease =
their reproductive success. In other parts of Europe, Pied Flycatchers =
reportedly prefer nestboxes containing old nests and fleas did not harm =
the flycatchers (birds may choose nestboxes on criteria other than =
parasite infestation).=20

Parasites that overwinter in nests of Cliff Swallows can cause that =
species to abandon nests, even in reproduction, and can cause premature =
fledging. Having worked around cliff swallow colonies, I have observed =
that these birds can be crawling with lice and mites; I would guess that =
it may be caused by the large numbers of birds in a small area allowing =
parasite numbers to build up.

Purple Martins prefer houses containing old nests. Eastern Bluebirds =
also prefer nestboxes with old nests, apparently because the old nest =
material houses larva of a parasitoid wasp (Nasonia vitripennis) that =
may reduce the numbers of parasitic blowfly larvae (Protocalliphora =
sialis). Blowflies are harmful to young bluebirds; parasitic lice and =
mites in bluebird nests have not been shown to have a harmful effect on =
the host, however.

The short answer *from me (not an expert)* is that I don't clean old =
nests from nestboxes very regularly. It seems that it may discourage =
some species, and - unless you have a known problem with detrimental =
parasites - doesn't make any difference to the bird.

Jon. Anderson
Olympia, Washington
festuca at olywa.net