|Dad makes a scrape in the snow|
How do birds detect changes in day length? Like many other creatures, they use photoreceptors – specialized cells that detect light and initiate a physical response to it. The photoreceptors in birds are not in their eyes (as they are in mammals like us), but deep in the brain, in an area called the ventromedial hypothalamus. The receptors react to light that penetrates birds’ thin skulls and surrounding tissues. Changes in day length (and possibly strength and angle, at least in non-equatorial latitudes) initiate major changes in birdie physiology and behavior.
To quote Scott Weidensaul, "Changes in the photoperiod pull many strings in a bird's body". As daylight length grows longer, the gonads of birds grow larger and produce more sex steroid hormones. This stimulates changes in behavior and sometimes plumage: depending on the bird, it may grow colorful plumage, choose and defend a territory, spend time vocalizing and flying to attract a mate, and engage in nest building. The Decorah eagles will begin rapidly responding to day length now that we are past the solstice. 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. We may see scraping as Dad did today in the snow: http://youtu.be/gsTEbNRGnTw. Copulation should begin in late January to early February, followed by egg laying in mid-to-late February.
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.
Some exploration points:
- Is there a predictable increase in frequency of nest visits as daylight length increases?
- Do Mom and Dad differ in their nest building behavior?
- Do Mom and Dad spend more time interacting as daylight length increases?
- Does weather also appear to impact nest-building activities?
Some reference materials:
- Living on the Wind: Across the Hemisphere with Migratory Birds (thanks, Joan!)
- Adaptation and evolution of photoperiod response systems in birds (http://www.springerlink.com/content/nx8804125624q526/)
- Explanation of the word Recrudescence (I didn’t know Senescence had an antonym!): http://en.wikipedia.org/wiki/Recrudescence
- Photoperiodism: Deep Brain Light Reception: http://www.nu-research.com/research/6169
- Photoreceptor Cell: http://en.wikipedia.org/wiki/Photoreceptor_cell