From Andrew on ALBirds about terns on Dauphin Island.
at this time of the year [April, 2019] any Least Tern with a black cap will also have a yellow bill, so I guess you are mistaking Sandwich Terns for them. Least terns are flying about the island now, and Sandwiches. Leasts are tiny – smaller body than a Sanderling when you see them sitting on the ground. If there are 2 or 3 of them flying together they are almost always noisy, with a sing-song high “deet de deet, deet de deet” call. Sandwiches have a harsh grating call, and are larger than Forsters & Common Terns. Sandwiches hold their wings like other terns, Leasts carry their wings very high, often in a high V giving them a distinctive flight appearance. Sandwiches have the typical complete black cap dividing the head between black and white at about the base of the eye level, Leasts have a distinct large white forehead that angles back on each side resulting in a black line through the eye onto the lores. Hope this helps.
Andrew Haffenden, ALBirds No. 5614