I had assumed these were Bonaparte's when I took the picture, but now finally cropping some for web use, I notice the legs of one looking much darker than the others. Is this a difference between an adult and young, or a different bird altogether?
Yep, both Bonaparte's - the bill is too small and dark for Black-headed, and the head pattern doesn't match Little Gull. The one with paler legs may be a first-winter (can't confirm without seeing its back!), but it could just be individual variation – the darker legs are more typical of a summer adult, but some individuals can show summer characters in winter or even all year*.
* In similar vein, I saw a Black-headed Gull a couple of weeks ago, with a full summer hood!
Thank you for the confirmation. That is interesting about the adult birds sometimes still showing signs of summer. It seems I am continually learning something new about the birds. They are definitely not boring. :)