Beak colouration of starling (Sturnus unicolor) males depends on the length of their throat feathers

This work was supported by the currently named Ministerio de Ciencia, Innovacion y Universidades and European (FEDER) funds (CGL2013-48193-C3-1-P, CGL2017-83103-P, CGL2017-89063-P). MAG was financed by a predoctoral contract (BES-2014-068661) from the Spanish Ministerio de Economia y Competitividad, and GT by the Ramon y Cajal Programme.