Answers are grounded in this paper and cite their sources.
Paul C. Mundinger; Annual Testicular Cycle and Bill Color Change in the Eastern American Goldfinch, The Auk, Volume 89, Issue 2, 1 April 1972, Pages 403–419, ht
No takes yet. Share an insight, caveat, or question.
Paul C. Mundinger (1972) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: