'Birds' covers about 8600 different species. Their sexual habits cover the full range of preferences.
Apart from the very odd occasion when a bird has had a traumatic experience or is associated with humans, they pretty much stick to their own species. Evolution adjusts specific alterations to species by small mutations in unison with their environment, not cross breeding.
Generally, species within the same genus can have offspring, but are mostly sterile so mating outside your own species is a waste of energy.
As for the odd bonk, most birds are broadly divided into two categories; monogamous and polygamous. The monogamous ones stay true to their one mate. The polygamous ones will have more than one mate and would think nothing of having a jump on one his neighbour's hens if he gets half a chance, as long as it is the same species.