I would say there are at least three kinds of hole size/entry tests. The validity of ALL rely on EXACT measurement of holes. The tests are meaningless without that. (Keith Kridler uses a Starrett Dial Caliper used by machinists to make sure the hole sizes are accurate.) They also depend on hole thickness and SHAPE (slot vs. oval vs. round vs. something else like flat on top or bottom or mouse hole style).
Natural preference for nesting. Before nesting, birds are presented with choices of boxes with various hole sizes, and select a box that has a certain hole size to build their own nest. Of course there are MANY other factors involved in nestbox selection, such as design, location, available choices, food availability, individual preferences, competition, maybe what kind of nestbox they successfully nested in the past or even were born in, etc. See Nestbox Pros and Cons.
Motivated choice. The bird is "driven" to enter the box.
The bird has a commitment to the box - e.g., a nest, or eggs or nestlings inside, and is thus motivated to enter. This motivation may overcome #1. This is what Frank Navratil used in his experiment - see below. He modified boxes AFTER the House Sparrows were nesting in them. This is why hole reducers used to protect native cavity nester eggs and young are usually placed on boxes AFTER an egg is laid.
In the case of a House Sparrow, this may also include a motivation to investigate and then prey or attack (eggs, nestlings or adults of another species) due to instinctive territoriality or aggression (see "Are House Sparrows "Evil"?)
In the case of starlings/magpies/jays etc. they may be motivated to get to a food source (eggs or nestlings). Of course if the hole is large enough or the nest shallow enough for them to reach the eggs via beak/extendable tongue (woodpeckers) they don't necessarily need to ENTER the box to prey.
Escape (Flight) choice. This is a test where a bird is placed in a trap, or inside a box, and in order to exit, must go through the hole. Most tests are conducted this way (it is much easier to control). If the bird is placed through the hole to start, this may introduce some bias, as they now "know" they can get through it. (Dogs are sometimes trained to go through doggie doors this way.) Bob Walshaw reported that he had to put duct tape over any ventilation slot that was more than 0.5" wide.
I would say this type of test probably provides the ABSOLUTE minimum size a bird could physically fit through, as they are VERY motivated to get out. It DOES "draw the bottom line" for what is physically possible.
Another factor that comes into play in Bergmann's Rule, a principle that asserts that body mass in warm-blooded animals can increase with higher latitudes and colder temperatures. Thus the potential for bigger birds in the North vs. South. More info at the pseudo-dictionary http://en.wikipedia.org/wiki/Bergmann's_rule (Warning: wikipedia is not the world's best source, as it often contains misinformation).
Frank Navratil's HOSP/Hole Size Tests - The Bluebird Box - see below
Minimum diameter for House Sparrow entry
By Frank Navratil
Back in 1995 I constructed wooden bird houses based upon drawings for a slot-entry style. These are very sturdy and easy to build. I still use some in the field, and bluebirds continue to nest successfully in them. A feature was that the 1 1/8-inch high horizontal entry slot would exclude House Sparrows.
I guess Chicago-area House Sparrows are somewhat smaller in size, because they readily nested in these boxes.
Curious as to what dimensions an entry must be to really exclude House Sparrows, I tried various sizes of round, horizontal, and vertical entry holes.
First, I allowed the sparrows to build nests and lay eggs in five wooden NABS houses with their 1 1/2-inch (3.8 cm) round holes. Now that the sparrows were motivated to re-enter the houses, I narrowed the openings day by day. (Sounds diabolical, doesn't it?)
I varied the entry size by screwing squares of 1/8-inch plywood with different hole sizes over the original 1 1/2-inch round entry hole. The slots were made by cutting two holes using the appropriate diameter hole saw, then carefully filing out the space between these holes.
1 1/2-inch by I inch (3.8x2.5 cm) slot still allows entry.
1 1/2-inch by 7/8-inch (3.8x2.2 cm) stops entry.
1 by I 1/2-inch (2.5x3.8 cm) slot still allows entry.
7/8-inch by 1 1/2-inch (2.2x3.8 cm) slot stops entry.
I was really surprised by the small opening required to exclude the sparrows, especially in the slot configuration. As far as House Sparrow motivation is concerned, it seems they are always motivated. I have watched sparrows try to enter a tiny wren house with a 1-inch (2.5 cm) diameter entry hole for days at a time. Not too bright, I guess, but based on their breeding success that persistence must be a good characteristic.
From The Bluebird Box, formerly at http://audubon-omaha.org/bbbox/nabs/fn1.htm
Reprinted, with permission, from "Bluebird," magazine (Winter 1999) 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 firstname.lastname@example.org or go to the NABS web site at http://www.nabluebirdsociety.org/
I have seen many trails "monitored" or unmonitored without passive or active HOSP controls, and the result is always the same. HOSP quickly acquire all the nestboxes, doing so with avian casualties.
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