> Birds are mostly monogamous This is quite misleading without further context. My understanding is that a majority of birds are observed to be "socially mono

> Birds are mostly monogamous This is quite misleading without further context. My understanding is that a majority of birds are observed to be "socially monogamous"; that is they form a pair lasting a single breeding season, and the pair works together to raise that season's brood. It's only a minority of species in which the same pairs reform across multiple seasons. I don't know how well understood this is; at least for sea birds, which are general among those to form long-term relationships, I've heard a theory that it might be related to the fact that fishing is an aquired skill that should improve with age; finding enough fish to feed both parents and chicks is difficult, and therefore there's a bigger risk if you change partners. The difficulty of finding food, and long time after birth before the offspring become self-sufficient, means these birds can typically only raise one or at most two young per year, so again the cost of failure is high. Even though in general the rate of social monogamy among bird species is high, this doesn't mean that birds are always parenting their own offspring. In some species (kittiwakes, terns) parenting others' chicks is really rare, but in other species there can be high rates of sexual infidelity e.g. some studies suggest that ~20% of chicks in flycatchers and ~30% in cardinals are unrelated to one (or, less commonly, both) parents. The latter may seem surprising, but of course brood parasitism (e.g. cuckoos) is well known, and there's a same-species variant of that in some species where a female will intentionally lay some eggs in the nest of another pair.