What a day! I started at dawn with Chuck-Will's-Widows and Eastern Whip-Poor-Wills cheek by jowl in the Francis Marion National Forest (S.C.), passed through a gauntlet of singing warblers, and by noon I was at the mouth of Murrell's Inlet at the north end of Huntington Beach State Park looking at a sandspit loaded with courting terns. The soundtrack of the video above is presented as a moving sonogram in the box at the bottom of the page.
Now I know when to expect Royal Terns to have a full black cap -- early April. They were the numerically, and probably acoustically, dominant species courting on the sandspit. Along with them were their smaller cousins, Sandwich Terns. They are the ones with the black bills (see video below). Both are in the genus Thalasseus (the name derived from the Greek work for "sea," and one of the most beautiful-sounding words on the planet to my ears). They are very similar in shape and posture, if not size. Also on the flat were numerous Black Skimmers and probable Forster's Terns, and a few gulls.
The sonogram will be a lot more meaningful if you have a copy of Nathan Pieplow's book in hand. Its full name is Peterson Field Guide to Bird Sounds of Eastern North America. My hat is off to Nathan for doing the years of research it took to assemble this masterful compendium and I especially appreciate his providing an entry to the sounds of seabirds to those of us who focus on landbirds. So, with my Eastern Pieplow open to page 172 I believe I detect Royal Tern peent and yap calls. Sandwich Terns have a very similar repertoire, and so we may be hearing them (too). The cut starts with a string of intense yaps , followed by an equally intense series of unidentified calls at 0:06. The first of several keer calls, the very familiar flight call, is at 0:16. Around 0:37 we encounter dry clicks that I would call kriks . An obvious peent is at 0:48. Now, having seen all of these, what shall we call the ones at 0:06? They seem partway between krik and peent to me. What a wondrous cacophony, and how handsome are the terns as they strut with their wings drooped and their crests flared! And by the way, thanks to this project, I just 10x'd my understanding of Royal Tern vocalizations.