whispered, "I never thought that Chandler euthanized the chick and the duck together. He was just as attached as Joey. I thought that, maybe, they just died of old age or sickness separately or close together... maybe one went to the "farm" first (particularly since we hear about the duck but not the chick in Season 7). It would strike me as odd if Chandler had euthanized the pets and then got Joey new versions of them."
That makes sense too, that is what I originally thought as well!