Mallards don't stay together for long and I'm not sure that many birds do? I think some swans are an example of birds that can pair for life.

Looking Mallards up on the RSP site, I see:

The role of the male is almost over once the clutch is laid. He remains sexually potent for a while in case a replacement clutch is needed, but gradually loses interest and joins other males to moult. At this time groups of males with no obvious duties often mate forcibly with females that appear to be unattached. This anti-social phase is short-lived and ends once moulting is underway.