The great entrance-hole debate
By Myrna Pearman,
Chairman, NABS Technical
Controversy is nothing new to bluebirders. Not scandalous
controversy, just good old-fashioned argument and animated but seldom
acrimonious wrangling. We've had raging debates over the merits of
different nestbox designs, about whether pairing increases or decreases
bluebird productivity, about whether fence posts should be recommended
for mounting boxes, and about which entrance hole size to use.
At the moment, we're arguing over whether bluebirds use "bird
houses" or "nestboxes" and whether "nest boxes"
is one word or two!
With the exception of box pairing, most contentious issues eventually
get settled after they've been argued about at meetings, or after
there's been a blizzard of letters, letters to editors and e-mails.
One of the longest-standing issues we've dealt with is in regards to
bluebird house/blue bird birdhouse/ bluebird nestbox/bluebird nest box
entrance-hole sizes. In the beginning, all was well in this department.
Bluebird conservation in North America focused on the Eastern Bluebird,
and there was a dramatic surge in public interest following the
publication of Dr. Larry Zeleny's book and his article in National
Geographic magazine in the 1970s. Almost everyone followed the designs
he recommended - a top opening box with a 4-inch by 4-inch floor and a 1
1/2-inch entrance hole.
When Art Aylesworth of Montana and Duncan Mackintosh of Alberta began
setting out Zeleny-style nest boxes in the late 1970s, they were
surprised to find that occupancy rate of Mountain Bluebirds was
relatively low, compared with the very successful trails operated by Al
Perry of Boise, Idaho, and the Brinkerhoffs in Washington. Art and
Duncan also noticed that the feathers in the shoulder areas of several
of the male Mountain Bluebirds occupying their boxes appeared broken or
Upon questioning the Perrys and the Brinkerhoffs, Art and Duncan
learned that 1 5/8-inch holes were being used exclusively on these
trails. When Art and Duncan returned home, both proceeded to enlarge the
holes in their boxes - Duncan to 1 5/8 inch, Art to 1 9/16 inch. The
results were outstanding, with occupancy doubling and even tripling in
some areas. By 1981, almost all the boxes in Montana, Alberta, and the
panhandle of Idaho had the larger openings. Occupancy continued to
increase. All was well.
Well, that is, until 1984, when starlings were first reported using
the boxes with the 1 5/8-inch hole. Since there were no reports of them
using the smaller 1 9/16-inch opening, Duncan and Art worked furiously
that season, retrofitting boxes with 1 9/16-inch patches. Their efforts
were rewarded with high bluebird occupancy and the total exclusion of
starlings. Based on these observations, Art and Duncan summarized their
findings in Sialia (1984). They stated in this article that nestbox
occupancy by Mountain Bluebirds would increase if boxes had entrance
holes larger than 1 1/2 inch, but that starlings were able to enter 1
5/8inch holes, so this larger size should not be used. They also
concluded that, while Western Bluebirds did not appear to require the
larger hole, they seemed to prefer it over the 1 1/2 inch.
Art and Duncan then began to lobby to have this larger hole size
officially recognized and accepted. Its adoption was swift and virtually
unanimous among Mountain Bluebird trail operators, but NABS was slow to
accept the change. Part of the resistance was because of Dr. Zeleny's
very valid concern that Bergmann's rule (species and races of birds
which live in colder climates tend to have larger bodies than their
warmer-climate relatives) may apply in this situation. if it did,
starlings in the southern parts of the continent might be small enough
to enter though this entrance hole size. He also was concerned that
natural selection might eventually favor smaller starlings. If smaller
starlings were able to exploit these hitherto unavailable nesting sites,
bluebird conservation efforts could be seriously compromised (personal
Despite this initial resistance, the 1 9/16-inch hole recommendation
soon appeared as the official hole size for Mountain Bluebirds. Although
there was never any official endorsement by a Western Bluebird
organization, the 1 9/16-inch size soon became the recommended size for
Western Bluebirds as well. The ready acceptance of the larger-hole
recommendation for Western Bluebirds was partly because of the wide
range overlap of Mountains and Westerns, and partly because it was/ is
widely believed that Western Bluebirds, like Mountain Bluebirds, are
slightly larger than their eastern cousins. But this may not be
accurate, as Dorene Scriven pointed out in a 1997 e-mail to several NABS
board and committee members. She checked with three major field guides,
and found that they disagree about the lengths of the three species, and
none discuss width!
For most of the 1990s, all remained calm on the entrance hole front.
Then, in November 1998, an email from the Hubert Prescott Western
Bluebird Recovery Group (HPWBRG) in Oregon was sent to NABS requesting
that it change the 1 9/16-inch entrance hole recommendation on their web
site to 1 1/2 inch. Starlings, the e-mail read, could utilize a box with
a 1 9/16-inch entrance hole.
As chair of the Technical Advisory Committee, I was disconcerted by
the mention of starlings using 1 9/16inch hole. If it was true, we could
be in trouble. Big trouble. I contacted HPWBRG, requesting both
clarification on the entrance hole size and confirmation about starling
use of the larger hole. Their reply confirmed that they indeed had
experienced no problems with Western Bluebirds accessing the 1 1/2-inch
entrance holes. They argued, understandably, that "if it ain't
broken, don't fix it." They did acknowledge, however, that their
documentation of starling use did not include an inspection of the exact
hole size on the offending boxes.
Art Aylesworth states that Montana Bluebird Trails has no - not one -
documented case of starlings using the 1 9/16-inch hole (personal
communication). And one can't argue with their sample size of some
15,000 boxes. In the few cases where starlings have used their nest
boxes, the entrance holes had been enlarged by squirrels or flickers. As
further proof that the 1 9/16-inch hole works, Art and his monitors have
documented cases of finding starlings stuck, dead, in the entrance hole.
There have been other studies conducted on this subject by using
captive starlings. In the Lehmann (1997) study, 10 starlings were placed
in boxes with 1 9/16-inch holes. Two escaped. The limiting factor with
these experiments, of course, is that while they do show that some
(panicked, desperate and perhaps gaunt) starlings can squeeze out of a 1
9/16-inch hole, they don't prove that plump, well-adjusted starlings can
So what is the answer? On the one hand, we can't refute the evidence
that the 1 1/2-inch hole works just fine for Western Bluebirds. Not only
has it worked well for the HPWBRP, it has worked well for Elsie Eltzroth
of Corvallis, Oregon, for the past 22 years (personal communication). On
the other hand, if the 1 91/6-inch hole also works - i.e. excludes ALL
starlings EVERYWHERE - wouldn't it be logical for us to advocate one
size for all? if, however, Dr. Zeleny's concern about the potential for
evolutionary forces to eventually favor smaller starlings, should we
first consult evolutionary biologists to comment on the potential for
Based on the extensive experience of Mountain Bluebird trail
operators, NABS recommends that the larger hole should be used wherever
there are Mountain Bluebirds, or wherever the range of the Mountain
overlaps with the other two (keeping in mind that ranges are dynamic and
usually change over time). The decision about which entrance hole to use
outside these areas is ultimately up the individual trail operator.
Aylesworth, A. 1984. Large box openings can be curse or blessing.
Sialia. Vol. 6, No. 4
Lehmann, D. 1997. Control tests to determine if European starlings
can pass through various hole sizes. Sialia Vol. 19, No. 4.
(Ms. Pearman can be reached at Site 2, Comp 2, RR 1, Sylvan Lake, AB,
Canada T4S 1x6.)
Reprinted, with permission, from "Sialia/Bluebird"
Journal of the North American Bluebird Society. NABS is a membership
organization for persons interested in bluebirds and other North
American birds which use cavities for nesting. For membership
information, send a message to
or go to the NABS web site at