I was wondering if many species of birds remain in their pairs during the winter?
Living near the coast I have several times seen two adult herring gulls together, and also crows on one occasion. Adult herring gulls are easy to tell from youngsters; so the pairs I saw couldn't have been a couple of this year's chicks still hanging out together.
When it comes to herring gulls, they usually form life-long pairs. They sometimes winter together, especially if the post-breeding dispersal is small. Thus the two birds you saw might be a pair.
When it comes to birds in general, different factors affect whether bonding is life-long or not, and whether pairs are wintering together or not:
Life-long bonding is thought to lead to greater breeding-success (hardly a surprising "assumption" that practising together is beneficial), so longer-lived species are more likely to form life-long pairs (not much use in forming a life-long pair if your mate is unlikely to be there next year). Another factor is the amount of post-breeding dispersal - in stationary species the birds are, well, stationary and thus don't "get lost". Some species who are both stationary and defend a territory also in winter are of course even more "bonded". In some species the familes the adults and their young spend the winter together (and in some cases join other families to form flocks as time goes on). If the species is gregarious in winter - well the pair could spend the winter "together", but also together with lots of other individuals. Some species split their familes early - especially waders where one parent leave as soon as the eggs hatch and the other leaves the young when fledged (thus more resources are left for the young which stay, feed and migrate later). And if a species is monogamous or polygamous is of course crucial. Much more can certainly be said about this, but I'm no ethologist, so I stop here.