“’Non-breeding’ plumage can mean any plumage that is not the plumage birds have moulted into specifically for the breeding season. (Some birds only moult once a year so at breeding time, their plumage is the same as the rest of the year, others have an additional moult early Spring into smarter feathers). So ‘non-breeding’ can mean an adult in winter plumage or an immature bird that has not yet reached maturity but as applied to birds that normally moult twice a year. Adult Birds moult out of their breeding plumage from around July onwards into a winter plumage. Usually it’s a lot more drab and less colourful than breeding plumage.” -Deb Burhinus

https://www.birdforum.net/threads/non-breeding-adults.402706/

Looks like they get into more detail but that was the first thing I found.

Is this like celibate monks or something?