The Time Traders by Andre Norton

Ross fought to break that hold, to turn his head, to face the peril which crept upon him now. Unlike anything he had ever met before in his short lifetime, it could only have come from some alien source. This strange encounter was a battle of will against will! The same rebellion against authority which had ruled his boyhood, which had pushed him into the orbit of the project, stiffened him to meet this attack.

He was going to turn his head; he was going to see who stood there. He was! Inch by inch, Ross's head came around, though sweat stung his seared and bitten flesh, and every breath was an effort. He caught a half glimpse of the beach behind the rocks, and the stretch of sand was empty. Overhead the birds were gone -- as if they had never existed. Or, as if they had been swept away by some impatient fighter, who wanted no distractions from the purpose at hand.