>...the "ideal" cavity size of 40 liters (0.4 m^3)
At least down here in the 'hot corner' of Kentucky, most of the cavities in bee trees that I've had the pleasure of knowing are much smaller than the "ideal" size - most would approximate the size of a volleyball.
This may be a reflection of our climate or could represent the best voids available owing to the relative lack of old growth forests.
I tend to find lots of these in older suburban environments and on the edges of woodlots.
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