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The great entrance-hole debate
By Myrna Pearman,
Chairman, NABS Technical
Advisory Committee

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 worn.

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 communication.)

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 enter.

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 future problems?

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.

literature cited:

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 membership@nabluebirdsociety.org or go to the NABS web site at http://www.nabluebirdsociety.org/ 


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date of last change 01/20/15Web space provided by the Audubon Society of Omaha.