From: drintoul@matt.ksu.ksu.edu (David A Rintoul)
Newsgroups: rec.birds
Subject: bluebirds and wrens: facts and opinions
Date: 11 Jan 1993 10:48:29 -0600
Organization: Kansas State University
NNTP-Posting-Host: matt.ksu.ksu.edu

First, some facts from the USFWS Breeding Bird Survey. (Letter and
data tables from Sam Droege to participants in the Neotropical Migrant
Workshop held in Colorado in the fall of 1992)

1. House wrens declined in Rhode Island for the period from
1982-1991.  This decline was NOT statistically significant.  Eastern
Bbluebirds were found on less than 14 BBS routes in RI; thus, no data
are available for this species in that state.

2. House wrens increased over the US and Canada over that same time
period.  Again, this difference was not statistically significant.
Eastern bluebirds increased over the US and Canada during that same
time period; this increase was also not statistically significant.

3. The cataloging of patterns in bird populations is a relatively
straight-forward exercise.  Determination of the environmental forces
that drive those patterns is a much more difficult proposition.

Now, from Andrew Taylor:

DR}2. House wrens are neotropical migrants whose population status may be
DR}stable at the present time.  However, given the rate of habitat loss
DR}in central and south America, it is not altogether unlikely that this
DR}species could decline over much of its range in the near future.  Why
DR}does Mr. Taylor think that well-meaning bluebird enthusiasts should be
DR}allowed to perhaps accelerate this process?

AT}I think these remarks are dubious for two reasons.

AT}If availablity of wintering habitat does limit the House Wren population
AT}then small changes to breeding success seem unlikely to have any effect
AT}I think its very implausible that the very small changes to overall breeding
AT}success that could result from disturbance of nests in Bluebird boxes
AT}could "accelerate the process".

I would be interested if there is some factual basis for this opinion.
I have searched for data on house wrens in this regard, and found
none.  If we resort to analogy (a favorite tool on this newsgroup), we
can find some instances where wintering habitat may be limiting
populations (e.g canvasbacks).  But analogies are, as mentioned
before, probably not relevant. If there is no factual basis for this
opinion with regard to house wrens, then perhaps the most prudent
alternative would be to not harass them during the breeding season,
which is what I advocated in the first place.

AT}My Nat. Geo. guide gives House Wren habitat as "brush and shrubs, orchards
AT}and farmyards, urban gardens, parks".  This does not sound like habitat
AT}which is being lost at a great rate.  It sounds like the common result of
AT}forest clearance in the tropics, so House Wren wintering habitat could even
AT}be increasing.  Am I missing something here?

Perhaps you are missing the distinction between breeding habitat and
winter habitat, which can be subtle or dramatic, depending upon the
species. I have found out that nearly all of the house wrens in North
America winter in Mexico.  House wrens found from Costa Rica south are
nearly all year-round residents there.  Does anyone have some FACTS or
REFERENCES regarding winter habitat choice in Mexico?  Does anyone
have some FACTS or REFERENCES regarding the rate of production or
disappearance of this habitat?  I discussed this with a resident
expert on wrens (E.D.  Kennedy, an ornithologist here who has worked
with house wrens at Rutgers and with Bewick's wrens here at KSU).  She
knew of no such data or studies.  Perhaps someone out there in
net-land could fill in this gap; perhaps not.  Again, in the absence
of real data, the prudent choice would be to leave these birds alone
on the breeding grounds.

DR}What if some other well-meaning bluebird enthusiast, say in Maryland, removes
DR}a Bewick's wren nest that he/she assumes was placed there by a house wren?

AT}A legitimate concern but hardly relevant to much of the area where Eastern
AT}Bluebird boxes are popular.

Data, please?  Where are eastern bluebird boxes most popular? 

Additionally, USFWS data indicate that the Bewick's wren is declining
across the continent.  These birds used to be COMMON in the
Appalachians; now they are extirpated from there.  Thus, present-day
range maps may not give a valid indication of the places where
suitable habitat for these birds exists.

DR}3. Simple expedients (such as providing more than one next box) allow
DR}the simultaneous nesting of both bluebirds and house wrens.

AT}This consumption of extra resources may reduce the number of nesting boxes
AT}supplied to Bluebirds and hence the effectiveness of the recovery program.
AT}If it doesn't fine.

Food resources or lumber resources?  Anyone who has operated a
bluebird trail knows that only a few of the boxes will contain
bluebirds.  It is not thought that this pattern is related to the
quantity or quality of the food resources.  In the absence of data, my
(equally valid) opinion is that there might be enough insects to
sustain many broods of birds, regardless of species.  Limitations on
reproduction of cavity-nesting species such as bluebirds and house
wrens usually revolve around nest-site availability, rather than food
availability.

Obviously the lumber resources are already spent, so why not let
another native species use some of the boxes?

CONCLUSION - Many opinions and no data support the contention that it
is OK (although illegal) to remove stick nests from bluebird houses.
Many opinions and some data support the contention that it is more
prudent (and fully legal) to leave these stick nests alone, and put
out more boxes if desired.  Make up your own mind; at least now
you have some facts.


-- 
Dave Rintoul                Internet: drintoul@matt.ksu.edu
Biology Division - KSU     Latitude 39.18, Longitude -96.34
Manhattan KS 66506-4901                         TCN: id2418
(913)-532-5832 or 6663                  FAX: (913)-532-6653
Newsgroups: rec.birds