Kevin J. McGraw, Geoffrey E. Hill, Riccardo Stradi & Robert S. Parker. The Influence of Carotenoid Acquisition and Utilization on the Maintenance of Species-Typical Plumage Pigmentation in Male American Goldfinches (Carduelis tristis) and Northern Cardinals (Cardinalis cardinalis). „Physiological and Biochemical Zoology”. 74, s. 843–852, 2001. The University of Chicago Press. DOI: 10.1086/323797.