Two of the smallest species of gulls that are regular in this province are Black-headed and Bonaparte's. The top three shots are of the former, bottom three the latter. They are fairly close in size but if you were to see them together you would notice that the Black-headed is slightly larger, 16 inches in length as opposed to 13.5 for the Bonaparte's. One of the most obvious differences especially from a distance is the dark under-wing of the Black headed. The bills are different as well with one having an all black bill while the other has a red bill with black tip. Otherwise in habits and appearance they are quite similar. If you could see the feet of the Black-headed ( I find it interesting that they are totally hidden here) you would see they are bright red. The Bonaparte's in these photos has pinkish feet but I understand they can be noticeably red as well.
The Black-headed was photographed on February 21 at Matthew's Lake. The Bonaparte's at East Ragged Island on February 27.