Storm Over Warlock by Andre Norton

Five days later they came up from the south so that this time Shann's view of the Terran camp was from a different angle. At first sight there had been little change in the general scene. He wondered if the aliens were using the Terran dome shelters themselves. Even in the twilight it was easy to pick out such landmarks as the com dome with the shaft of a broadcaster spearing from its top and the greater bulk of the supply warehouse.

"Two of their small flyers down on the landing field. " Thorvald materialized from the shadow, his voice a thread of whisper.