The smaller tern in the foreground is a breeding Common Tern and the larger terns in the background are breeding Caspian Terns.  You can see how much smaller the Common Tern (about 12") is compared to the Caspian Tern (about 21").

BREEDING COMMON T÷

BREEDING CASPIAN TERNS, WINDERMERE BASIN, HAMILTON, ON

MALE TREE SWALLOWS, WINDERMERE BASIN, HAMILTON, ON

HOUSE SPARROW, HIGH PARK, TORONTO, ON

CANADA GEESE, GRENADIER POND, HIGH PARK, TORONTO, ON

MALLARDS, NORTH DUCK POND, HIGH PARK, TORONTO, ON

   In the first photo the male Mallard is on the left and the female on the right. The second photo shows a male Mallard.