Eagle beastmen's vision was definitely the best in the world of beastmen. Even when they were flying high up over 10,000 meters in the sky, they would be able to accurately discern the animals on the grass plains. Even the slightly bigger butterflies and wild snakes wouldn't be able to escape from their eyes.
Moreover, Muir had already arrived at the borders of the oasis and was just several thousand meters away from the lake. At one glance, he saw the shaking tree leaves, and there was a hint of yellow color inside that was about the same color as that of sand.
However, although the hint of yellow was in the dark, it showed a vague golden color. That was not the color of sand. Muir concluded that an animal was definitely inside.
Food was rare in the desert, and if they were to encounter prey, how could they not hunt it?