Fig. 1. Field data on seasonal changes in length of the left testis (circles) and cloacal protuberance (CP, triangles) in free-living pine siskins, Cassin's finches and gray-crowned rosy-finches breeding at Tioga Pass, California. The line indicates the number of hours of daylight. The hatched bars indicate when most birds of each species were exhibiting feather molt. Testis length was measured in a subset of the birds for which we measured CP. Sample sizes are: pine siskin: 207 (CP), 31 (testis); Cassin's finch: 574 (CP), 82 (testis); gray-crowned rosy-finch: 31 (CP), 6 testis.