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I'm afraid I do not see any "intruder".
Maybe - the fifth one from the left at the upper row (longer neck)?
The rearmost bird?
Can you find out its specie?
Not a clue!
Looks like to me there is some Teals , Mallards , and Gadwalls mixed together.
Juvenile recognition is sometimes very tricky Brute but these are all mallards — juveniles and matures females — but the one!
OK ... But the one with the black beak sure looks like a Gadwall to me.