Beak color variation (IMAGE) Uppsala University Caption Beak color variation in different bird species. Above left, a Medium Ground-Finch fledgling with a yellow lower beak begs for food from its parent. Above right, a Yellow-billed Cuckoo. Below left, a Yellow-footed Gull with a bright yellow beak and legs. Below right, a Brown-headed Paradise Kingfisher with a bright red beak. Carotenoid pigments form the basis for red and yellow color in bird beaks and in Darwin's finch nestlings, a genetic mutation interrupts a key carotenoid processing gene, leading to a bright yellow beak. Credit Tui de Roy – Roving Tortoise Photos (finch) and Erik D. Enbody. Usage Restrictions May only be published in connection to reports about the research by Enbody et al or after permission from photographer. License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.