Birds don't live as long as people do, nor do they have complex social structures. It'd be more fair to compare humans to other primates, though I don't have those numbers on my desk right now.
As far as primates go studies have actually shown that the amount of adultery in a species is linked to the size of the testes. Gorillas for example have (iirc) the smallest testes relative to their size, since they have a secure harem to mate with and the chance of being able to sneak in and mate with a foreign group is relatively low. Chimpanzee on the other hand have really large testes relative to their size and other primates, and adultery is common in their societies. The reason is obviously that the more sperm they can inject into as many females as possible the greater the chance that the female will bear their child. Whereas the gorilla is relatively safe with carrying a smaller load since there's not much of a risk that anyone but him would mate with any in his harem.
Finally, humans have testes that are somewhere in these extremes in size and one can as such draw the conclusion that adultery is relatively common in humans but not completely widespread.
If Dwarves were to follow this pattern we thus only need to measure the size of their testes compared to their size to be able to come up with a hypothesis about how common adultery is in dwarf society ^^
(I'm retelling this from something I read a few years back, but I'm fairly sure I remember it correctly
)