Where both species occur, Common Tern and Forster´s Tern are sometimes very difficult to distinguish, especially at the time they change plumage.
The differences are usually the following:
- Common Tern usually has a darker mantle and primaries;
- Forster's Tern usually has clean white underparts (Common Tern is much greyer);
- In Common Tern, the wingtips typically look slightly darker than the wing coverts;
- Forster's Tern usually has longer and more red-orange legs;
- adult non-breeding and 2nd year Common Terns show a dark carpel on the folded wing (shoulder) which is absent on Forster's Tern.

I think the bird on the photos are Common Terns but I am not 100% sure (in the company of a Royal Tern (large orange bill) and Sandwich Terns (black bill with yellow tip).

North Beach, Fort de Soto County Park, Tierra Verde, Florida, USA, 31 March 2022