Why did Albert whistle to Joey?

Albert whistled just to be sure it was Joey....

‘It could be,’ said Albert quietly. ‘You’re right David, it could be him. It sounds like him even. But there’s one way I know to be sure,’ and he untied my rope and pulled the halter off my head. Then he turned and walked away to the gateway before facing me, cupping his hands to his lips and whistling. It was his owl whistle, the same low, stuttering whistle he had used to call me when we were walking out together back at home on the farm all those long years before. Suddenly there was no longer any pain in my leg, and I trotted easily over towards him and buried my nose in his shoulder. ‘It’s him, David,’ Albert said, putting his arms around my neck and hanging on to my mane. ‘It’s my Joey. I’ve found him. He’s come back to me just like I said he would.’