Wild birds show cyclical alternation between breeding seasons and seasons of sexual inactivity. During the latter the sex glands are at rest and are very small. With the approach of the breeding season they start to grow rapidly, -- increasing in volume up to one thousand times. Accompanying this growth of the gonads many secondary sex characters also show a characteristic development. This is true in first respect for the sperm- and oviducts, but also external secondary sex characters often show striking changes. The bill color of the sparrow is light brown during the resting period and becomes blue black in the breeding season. In some birds, as the Rose-breasted Grosbeak, a remarkable change also takes place in the plumage. Experiments have been performed and will be presented to show the external and hormonal factors that control these seasonal changes.
Proceedings of the Iowa Academy of Science
©1934 Iowa Academy of Science, Inc.
"Seasonal Variation in Birds,"
Proceedings of the Iowa Academy of Science, 41(1), 314-315.
Available at: https://scholarworks.uni.edu/pias/vol41/iss1/113