Extraocular light perception in photoperiodic responses of the White-crowned sparrow (Zonotrichia leucophrys) and of the Golden-crowned sparrow (Z. atricapilla)

Abstract

Testicular maturation, migratory restlessness (Zugunruhe) and fattening are induced in many species of birds by long day-lengths in Spring. We have tested the hypothesis that extraocular photoreceptors located in the brain are involved in mediating these photoperiodic responses in White-crowned sparrows (Zonotrichia leucophrys) and Golden-crowned sparrows (Z. atricapilla). Our approach consisted in reducing the amount of light penetrating to the brain with either black India ink injected under the skin of the head (Golden-crowned sparrows) or by covering the entire head (except eyes and beak) with a black collodion hood (White-crowned sparrows). Birds treated in these ways showed significantly less testicular growth, Zugunruhe and premigratory fattening when placed under a 16L-8D photoperiod than control birds which did not have their brains shielded from light. However, even when the bird brains were shielded from light and although light intensity was close to threshold level, some testicular growth, Zugunruhe and fattening did occur. We conclude that extraocular photoreceptors are involved in the control of the three photoperiodic responses studied, but that the eyes are possibly of significance as well.