Little Tern versus Saunders's Tern
From The UAE Checklist:
Little Tern Sternula albifrons
Common passage migrant (some inland) March to May, uncommon or rare in autumn (but when separation from Saunders’s Tern problematic); no definite records January or February.
Saunders's Tern Sternula saundersi
Fairly common breeding visitor (300-500 pairs; Jennings 2010) and passage migrant, scarcer or absent in winter.
Difficult to separate from Little Tern unless wing photographed.