In North America most birds form bonds for at least a single nesting. These pairings allow birds to split domestic duties for protecting eggs and caring for hatchlings.
Other pair bonds include mating for life, either by pairing up again each breeding season or remaining with each other year-round. Cardinals, jays, doves, and robins are some of the common backyard birds that spend several seasons together with the same partner.
Even cowbirds which lay their eggs in other birds nests are largely monogamous.
One exception to the social pattern of monogamy with backyard birds that comes to mind is the Ruby-throated Hummingbirds. After a brief courtship and mating, the female builds a nest and raises her family alone. Male hummingbirds do not help raise the young.