Little Tern vs Saunders's Tern
From the UAE Annotated Checklist:
Little Tern Sternula albifrons
Common passage migrant March to October, some recorded inland.
Status in autumn and winter not clear, when separation from Saunders’s Tern problematic. No definite records January or February.
Saunders's Tern Sternula saundersi
Fairly common breeding visitor to Arabian Gulf islands (300-500 pairs; Jennings 2010) and passage migrant, scarcer or absent in winter. No inland records.
Can be difficult to separate from Little Tern outside of breeding plumage.
A good start is reading this article from Birdingfrontiers
As well as this article from British Birds
This link to breeding Little Terns from Egypt from Radioactiverobins
UAE photos of Saunders's Tern - here
UAE photos of Little Tern - here
UAE photos of non-identified - here
Little Tern
It is currently thought that most Little Tern in the UAE can only be positively identified as adult in breeding plumage, when the following features are seen:
- white forehead ending in a supercilium reaching behind eye
- all white rump, distinctly paler than the mantle. Rump can have a greyer centre, or even all grey as seen in the UAE and further east.
- Only 2 outer primaries black, with white primary shafts.
Non-breeding adults very similar to Saunder's Tern, possibly slightly paler.
Immature birds virtually identical to immature Saunders's Tern. First-winter plumage acquired in October, when identical to non-breeding adults.
Call is normally a rasping 'kriet'. Check xeno-canto
Saunders's Tern
It is currently thought that most Saunders's Tern can only be positively identified as adult in breeding plumage, if the following features are seen:
- white forehead with straight border, not reaching behind eye
- all grey rump, same colour as mantle
- 3 to 4 outer primaries black, with black primary shafts (this can be difficult to see)
Non-breeding adults very similar to Little Tern, possibly slightly darker.
Immature birds virtually identical to immature Little Tern.
Call is stronger than Little Tern, disyllabic, resembling Common Tern. Check xeno-canto