The Decorah Bald Eagles

Many animals monitor the length of the day in order to keep track of the seasons. This phenomenon, known as photoperiodism, allows their physiology and behavior to adapt to seasonal changes in the environment, and indicates the most favorable time of year to produce offspring. Although bird species vary in their responses, the annual cycle of birds overall is driven by daylight length.

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 manages to penetrate 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.

So how does daylight length affect birds? As daylight length grows longer, the gonads of birds will grow larger, they will produce more sex steroid hormones, and they will reproduce. The Decorah eagles in particular will begin to spend more time working on their nest. Although they are still at the fish hatchery, we haven’t seen much of them lately. But as the day grows longer, they will spend more time bringing in sticks for the outer cup and some softer materials, including corn stalks, for the inner bowl. Over time, this courtship practice may cause their nest to become very large. Right now, the Decorah eagle nest is roughly the size of a bathtub that somehow became lodged in the upper branches of a cottonwood tree. As bald eagle nests go, it is probably around average. The biggest bald eagle nest that I can find reference to is in Florida. That nest was 6.1 meters deep, 2.9 meters wide, and weighed 2,722 kg (almost 3 tons).

Birds may also respond to changes in daylight length by migrating. A very small number of species do not migrate at all. Some species are complete migrants – that is, all populations of a species migrate. Complete migrants in Minnesota include loons, rosy breasted grosbeaks, and prothotonary warblers. Many birds are partial migrants – some populations migrate, and others do not. The long list of partial migrants includes bald eagles and peregrine falcons.

Picture this: the Decorah eagles will begin rapidly responding to day length now that we are past the solstice. They will begin by increasing the frequency of nest visits and working on their nest. As the days get longer and they have eggs and then eaglets to tend, the eagles will stop responding to daylight and their gonads will shrink. Once the young eagles are on the wing, the entire eagle family will decrease visits to the nest until the adult eagles start up again next year.

Some exploration points:

• Is there a predictable increase in frequency of nest visits? Watch the stream or use the eagle dailies to see how often the eagles show up in the nest, and plot the results.

• Do the male and female eagles differ in their nest building behavior? The female is bigger than the male, so look at size to determine which is which.

• What is migration? When a peregrine falcon leaves Minnesota every year to go to South America, it is clearly migration. When a bald eagle flies over to the fish hatchery, it is clearly not migration. But what about birds that annually move just a few miles, or that wander in a fairly local area? I will be writing more about migration later on.

• How do local conditions like food and water availability impact migration? Presumably, insectivores in Minnesota would be complete migrators, since their food supply vanishes in the winter. I would think ducks would vanish as well, but I saw some in Minneapolis earlier this week. I saw a vast flock of robins – I mean huge – at the Walker Art Center’s sculpture garden on December 22. So much for the robin being the first sign of spring!

• Do non-migrators and complete migrators have different physiological responses than partial migrators?

I would love to hear from any teachers that end up exploring any of these points in the classroom. Email me at ries93@gmail.com and let me know about it, or friend us and post it to our facebook page:
http://www.facebook.com/pages/Raptor-Resource-Project/103786266324668

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 (yes, I use Wikipedia): http://en.wikipedia.org/wiki/Photoreceptor_cell

And I listened to the Cee-Lo Green song ‘old-fashioned’ on YouTube while writing this up.