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 http://audubon-omaha.org/bbbox/nabs/fn1.htm. 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).
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