But your guess is not a bad one, for all Terns are also called Sea Swallows, because of their dashing flight.

Both Gulls and Terns nest on Gull Island, where Olaf is going to take us some day when the water is smooth.

The storm has driven some of them into the bay, where they do not usually come until later in the year; but in winter great flocks of Gulls live about our beach, clamming on the bar at every low tide." "I guess we had better tie up yonder," said Olaf, when they had gone a couple of miles up the river.

"And then I can put the children in the little boat and pole them in among the reeds." So the Doctor and Olive went ashore, where the sharpie was tied to the end of what had once been a small wharf, while Dodo, Nat, and Rap crouched down in the dingey, obeying Olaf's order to keep very still and not make the boat tip.
The little reed-bordered creek that they entered was quite narrow, and soon grew to be only a thread of water, where they could touch the reeds on both sides.

They heard many rustling sounds, but for some time could see nothing.