The Whiskered Tern can be confused with the Black Tern during the non-breeding season. In this case, the Black Tern one has darker tones in the lesser secondary coverts and the most external primaries. During the breeding season, the Whiskered Tern is unmistakable due to its black cap.
Both sexes have a very similar plumage that makes it very difficult to sex this species.
It is possible to recognize 3 different ages:
Juvenile specimen: they lack the light-colored collar on the nape. The light gray feathers of the back, scapulae, and infra-coverts have black and sepia subterminal bars.
2nd-year specimen: they are very similar to the adult but indistinguishable after the last molt in which they retained juvenile feathers. A small percentage is recognizable during the breeding season for not starting the breeding plumage.
Adult specimen: during the breeding season (summer plumage) they have a fully dense black cap and nape. During the fall, they acquire a light collar on the nape. On the other hand, the light gray feathers of the mantle, scapulae, and infra-coverts are uniform in color and lack subterminal bars.
Whiskered Tern undergo a complete post-breeding molt in which they change almost all of the body feathers and the outer primaries (P7-P3) in the breeding areas. At this point, they suspend the molt that is completed in wintering areas after the autumn migration. Juvenile specimens undergo a complete molt that generally starts in breeding areas with some body feathers and outer primaries, then suspends the molt for the autumn migration and finishes in the wintering areas. Both ages perform a partial pre-breeding molt in which they molt most of the body feathers and some of the inner primaries.
Primaries: 10 per wing.
Secundaries: 15-17 per wing.