Induction of Testicular Development in House Sparrows, Passer domesticus, and White-Crowned Sparrows, Zonotrichia leucophrys gambelii, with Very Long Days and Continuous Light

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Abstract
Two models of the mechanism that measures daylength in the photoperiodic testicular response in Passer domesticus have been proposed. Both explain adequately the "ultra-short-day" response of this species, which appears to be unique among avian photoperiodic species. However, the models differ in their predictions of the response to very long days and to continuous light. Because of this we have examined experimentally the responses of this species thereto and have compared them with those of Zonotrichia leucophrys gambelii, which does not respond to "ultra-short days." We measured testicular growth rates and plasma levels of luteinizing hormone (LH), both of which increased with increasing daylength up to about 18L 6D and were maximal for all photoregimes from 20L 4D through LL. The results are consistent with the predictions of the simplest form of external-coincidence model, but not with those of the internal-coincidence model of Dolnik or the class of simple models that involve coincidence in phase of dawn- and dusk-entrained oscillators. The latter predict a less than maximal response to very long days which is inconsistent with the results of our investigation.