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For birds, as days grow longer, gonads grow larger

The Decorah eagles spent the day nestorating, vocalizing, gently pecking, and deciding where exactly that stick should go.

Photoperiodism- the phenomenon which allows birds and other animals to adapt to seasonal changes in their environment through their physiology and behavior, and signals to them the most favorable time of the year to produce offspring. Basically, it is Mother Nature signaling the change of seasons... daylight length is one factor.

Birds, like many other creatures, use photoreceptors to detect changes in day length. Photoreceptors are specialized cells that detect light and initiate a physical response to it. In mammals, photoreceptors are in our eyes. In birds, this is not the case. Photoreceptors are deep in their brains... in an area called the ventromedial hypothalamus. "These receptors react to light that penetrates the birds' thin skulls and surrounding tissues. Changes in day length (and possibly even strength and angle, at least in non-equatorial latitudes) initiate major changes in birdie physiology and behavior," according to blog from Raptor Resource Project.

Among Raptor Resource Project's projects is the full coverage of a pair of eagles at Decorah, Iowa. The project includes a nest cam and a network of "followers" of eagles who live in the Decorah area.

To quote Scott Weidensaul, "Changes in the photoperiod pull many strings in a bird's body".

As days grow longer (well, as there is more day light in a day's time), the gonads of birds get bigger and produce more sex steroid hormones. This stimulates changes in behavior. For instance, in some birds, this means choosing and defending a territory, spending time vocalizing and flying to attract a mate, and engaging in nest building. It may also mean changes in feather plumage... for instance more colors or more distinct existing colors.

"As the days grow longer, the eagles' gonads will swell in response. They will move deeper into courtship and nest-building activities will increase in frequency, duration, and intensity. Mom and Dad will spend more time bringing in sticks for the outer cup and crib rails, and softer materials, including corn stalks, for the inner bowl."

"Daylight hours will exceed nighttime hours on March 20th, although the eagles will have already experienced a new set of hormonal changes brought on by egg-laying and eaglet care. After June 21st, daylight hours will begin decreasing, which will trigger another set of biological changes in many birds. By late summer and early fall, birds might begin moving away from natal or home territories, packing in calories, gathering or flocking together in larger groups, including mixed age and species flocks, and in many other ways preparing for migration and overwintering. This year's young eagles will disperse and we'll see less and less of the adult eagles until early winter, when the cycle will begin anew."

We, as humans, can observe when eagles are getting ready to have children. Courtship and bonding rituals signal the nearness of reproduction. "We see Mom and Dad both actively getting the nest ready by the addition of branches, twigs, corn husks and stalks, and other soft materials to line the nest bowl," writes Raptor Resource Project. Other behaviors observed at the beginning of mating season include bill stroking and pecking (kisses), vocalizations, pair perching, and some footing or body stroking. Sweet!

Here is a video from Raptor Resource Project's nestcam showing some romance budding between two adult eagles. Baby, oh baby.

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