The male's yellow beak differentiates it from the female.

雄鸟黄色的喙是与雌鸟相区别的主要特征。

《牛津词典》

Plush Penguin with Igloo Carrying Case: The plush penguin is black and white with a yellow beak and feet;

毛绒企鹅玩具和可携带的圆顶冰屋：毛绒企鹅玩具为黑白相间，嘴和脚为黄色，并有硬塑料制的眼睛。

youdao

The chickens are characterized by white plumage, single comb, red ear-lobes with white spots, yellow beak and shanks.

成都白鸡全身羽毛白色，单冠，耳叶红色带有白斑，喙、胫黄色。

youdao

Just below his eyes is a yellow beak that opens and closes as it speaks.

ENGADGET: Furby gets a reboot for 2012, we go hands-on (video)

And on the 500-peso notes, designers gave the island's famous blue-naped parrot a yellow beak, instead of the red beak that Mother Nature gave it.

WSJ: Canada's New Banknotes Strike Some as Loonie

He opened the coop and stuck his hand in toward the fierce creature inside, eyes yellow, beak sharp as a cat's claw, trilling a warning so low it was almost a growl.

NPR: Excerpt: 'The Prince of Frogtown'