Bald Eagles

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REPRODUCTION & HORMONES

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Summary: Reproduction and Hormones

Reproduction in birds involves mostly the same or analogous hormones as in humans and other mammals, but the processes involved are affected by some significant differences between birds and humans:
- Birds lay hard-shelled eggs.
- Most bird species reproduce every year.
- Birds’ gonads shrink and cease to function (gonadal regression) after the Reproductive stage and must regrow at the beginning of the following year’s Reproduction stage (gonadal recrudescence).
- Birds’ Reproduction is significantly affected by exogenous cues from the environment, especially the changing photoperiod.
The main Reproductive hormones are produced by the Hypothalamus, the Pituitary Gland, and the Gonads (Ovary and Testes), grouped together as the Hypothalamo-Pituitary-Gonadal Axis. The Adrenal Glands and the Thyroid also secrete hormones with a role in production.
The Reproductive stage in a bird’s Life History proceeds through three phases, each of which involves a complex and systematic program of hormonal secretions (described in the full document):
- Preparation, which includes territory establishment, nest-building, pair-bonding, gonadal recrudescence, sperm production, yolk deposition, ovum development, and the beginning of brood patches.
- Maturity, with two sub-phases:
  - Sexual, when sexual interactions between female and male become more frequent, the nest is finalized, Gonads mature, brood patches continue to develop, eggs develop, ovulate, are fertilized, and are laid. The embryos slowly grow, and gonadal regression begins.
  - Parental, when parents incubate eggs and brood chicks, and gonadal regression proceeds. Molt begins about halfway through incubation. Parents engage in fewer intense sexual interactions.
- Termination, when juveniles fledge and disperse, adult Molt proceeds, gonadal regression is complete, and adults transition into a Movement stage.
Reproductive behaviors, hormonal secretions, and physiological changes work together, each affecting and reinforcing the others during each of the phases in a continuous feedback loop.
The mutuality of behavior, secretions, and physiological changes is key to establishing synchrony between female and male, which is essential to ensure successful breeding.
Prolonged disruptions that occur at any time during the Reproduction stage, including extreme weather, territorial challenges, loss of a mate, nest, or food supply, and human disturbances, can interrupt hormones and behaviors enough to cause breeding failure.

HOME
BLOG
BIOLOGY
BREEDING
EAGLET GROWTH
- EAGLET DAILY PHOTOS & MILESTONES
- REFERENCES
BALD EAGLE NEST CAMS
GLOSSARY

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