I am much indebted to Dr. Peter Marler for his advice while this work was in progress and for his valuable criticism of the manuscript. Thanks are also due Dr. William Fish for the recordings of wild grosbeak songs and Dr. Peter Winter for the German summary. This research was supported by a grant to Dr. Marler from the National Science Foundation. The Schering Corporation generously supplied the testosterone.
Max-Planck Institut, 8 München 23, Kraepelinstraz̀e 2, Germany. As of Sept. 1, 1965: Department of Zoology, University of Wisconsin, Madison, Wisconsin.