"Listen!" whispered Audubon softly. "Isn't it like a goldfinch singing under its breath? The same little trills, the same sustained sweet notes in between—but it's a vireo, Norton! Would you guess it?"
Norton stared, but his friend was in deadly earnest. He heard the hidden bird change its song suddenly, and Audubon gripped his arm hard.
"There—that's the real vireo song, with the pulse-like, clear-cut notes! Did you ever hear such a thing before? That bird was giving a real goldfinch trill, man——"
"Where's the cottonwood?" broke in Norton drily. The other looked at him, his face blank on a sudden.
"Why—why—I clear forgot——"
Norton laughed, but Audubon suddenly gripped his arm harder. The bird above had broken off in mid-song, for no apparent reason. From somewhere outside the covert came the rattle of a stone, followed by a horse's snort.
"Keep quiet," breathed Norton, looking into the deep eyes of his friend. "Someone's on the trail."
Audubon nodded, and the two men stood tense and motionless, every nerve on edge. With startling abruptness there came a deep curse from the trail.
"Damn it, pull up! I ain't no Injun!"
There followed a chuckle in another voice—one that sent Norton's hand to his rifle.