Hola Rodrigo, gracias por comentar y te copio textualmente del tomo I de Hanbook of Birds of the world ( Del Hoyo, J. et al edit., Linx Edicions, Barcelona 1992) " The patch of bare skin om the crown of thr recently hatched chick acts as a signal spot to indicate to the parents when it is hungry, alarmed or need atention. Normally when the chick is untroubled and satified the patch is pale yellow, but it changes colour to bright red, when it is excited or hungry". (pag183). Sobre el comer plumas cito: "Grebes are unique in the habit of eating their own feathers..." y sigue. Espero haberte aclarado las dudas, y cuando quieras puedes venir a consultar en mi biblioteca. Un cordial saludo