The eclipse plumage of the male mallard (Anas platyrhyncha platyrhyncha) which normally appears in June and July has been produced prematurely in February and March by subjecting the birds to artificial light in addition to normal daylight.

Castration of male mallards did not prevent the assumption of eclipse in the first year but did so in the second year. Regenerated tissue was not found. It is concluded that the eclipse plumage is not caused by the direct action of a testicular hormone on the feather follicle, but that removal of the testis sooner or later produces an effect on some other endocrine organ or organs which consequently ceases to be sensitive to seasonal change (light) or fails to produce the hormones which bring about eclipse.

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