Where do falcons and eagles go in winter?

Lately I've been getting asked where our falcons go to winter. The answer is complex. Birds do not fly south simply to escape the cold. Snow and ice seal the food supply and/or habitat of many birds away until spring: hummingbirds don't have flowers, waterfowl don't have water, and most insectivores don't have insects. However, peregrine falcons and bald eagles are able to weather winter since their food supply remains locally abundant, or at least present, year-round. Both species are partial migrators: that is, some birds migrate and others don't.

Bald eagles eat a wide variety of food, including other birds, mammals, reptiles, and invertebrates. They take their food live, freshly killed, frozen and left in the nest, or as carrion, and they can gorge food quickly and fast for days if need be. Peregrine falcons eat almost solely other birds they catch in flight, but many birds winter in Minnesota and Iowa, especially in cities and along open stretches of water. While there has historically been some natural open water along the Mississippi river during the winter (Eagle Valley, for example), the number of open water spots has increased with industrialization. Power plants discharge warm water that keeps stretches of rivers and lakes ice-free, providing spots for birds like swans and geese to winter (especially when people feed them, as they do here). Cities and industrial sites provide food and shelter for non-migratory birds like rock doves, which builds populations and makes wintering in place much easier. Even in the harshest winter, falcons and eagles have ample access to food in places like Minneapolis, St. Paul, Bayport, Decorah, Red Wing, and even northerly Duluth. 

Even with ample access to food, our winter weather is pretty gnarly. Birds have wings, so why don't they leave? We don't know for sure, but we do know that migration is costly and risky, and a good territory is hard to find. Falcons and eagles that stay on site have an advantage against territorial interlopers and are less likely to find themselves replaced by another mate come springtime. Years of occupancy on site yield a deep map of the the territory - places to find food, niches to shelter, and a knowledge of the local fauna - that helps animals survive even under adverse conditions (see this blog for more information about how eagles handle winter). 

So I've talked about peregrines and eagles that stay, but what about peregrines and eagles that leave? Transmitter studies indicate that mates don't migrate or spend time together off their breeding grounds. Among falcons and eagles, young don't migrate with parents, and we don't think they spend much time together after they disperse. 

Bald eagles tolerate - maybe even like? - cold. Many eagles that migrate don't fly into latitudes free of ice and snow. Eagles in central Canada might shift south as far as the Mississippi river in Minnesota, Iowa, and Wisconsin. Alaskan eagles go west to the sea. Eagles in central north America might fly to an open body of water not too distant from their breeding grounds, joining winter eagle congregations in places like Eagle Valley, Wisconsin; Clinton, Iowa (Eagle Point Park); Bayport, Minnesota (south of Xcel Energy's Allen S. King plant); and many other places.

Peregrine Falcon Migration Map

Peregrine falcons have been documented traveling a bit further south. We've had band returns from south Florida, an oil rig in the Gulf of Mexico, and Costa Rica (click these links to read about Island Girl and Immaculata). Thousands of Tundra peregrines have been trapped and banded at the Padre Islands in south Texas as they fly from an area up above the arctic circle to Central and South America - a distance of up to 15,500 miles! Florida reported a world record number of migrating peregrines just last week - an amazing 1506 in a single day count! 

In short, migratory eagles from Minnesota, Iowa, and Wisconsin might easily be wintering fairly close to home, spending time on open stretches of water along large rivers with eagles from central Canada who flew several hundred or a few thousand miles to get there. D1, D14, and Four all wintered in NE Iowa and (in D1's case) SE Minnesota (explore the cluster map for details). However, peregrine falcons from the same area might fly thousands of miles south to warm their feathers in Central and South America before returning to our area in late February and early March. 

The whole question of photoperiod length becomes even more interesting when migration is factored in. In late February, our latitude gets around 11 hours of sunlight daily. We peak on June 21, at 15 hours, 36 minutes. Non-migratory birds and Canadian migrants to our area also experience the shortest day, at 8 hours and 46 minutes on December 21. Falcons that migrate to Costa Rica experience a different photo-year than their non-migratory counterparts, however. A falcon that left on October 1 has a shortest day-length of around 11 hours. Its days will get slightly longer as it wings south. The shortest day in October in Costa Rica is 11 hours and 46 minutes long. The shortest day of the year is 11 hours and 32 minutes long. These birds have more average daylight over the course of the year, and a much flatter photo period map during the time they spend near the equator. The difference is even greater for birds like the Tundrius falcon, which summers at and above the arctic circle.

Some things that helped me write this post: 

• Mandernack satellite telemetry study: http://www.bioone.org/doi/abs/10.3356/JRR-10-77.1
• The Peregrine Falcon Research Group study: http://www.frg.org/SC_PEFA.htm
• Time and Date: http://www.timeanddate.com/

Why Are The Eagles Working On The Nest?

2016 note: The winter of 2014 was mixed. To date, the winter of 2016 has been far warmer than normal. Click here for a synopsis of 2014/2015 - the season this blog was written for. 2016 is starting much like 2014, so it will be interesting to see how we finish.

Why are the eagles working on the nest? Are they predicting a hard winter? Does nestoration frequency or intensity signal a snow and cold yet to come? These questions have turned up in a few places recently. It appears that the recent flurry of nest activity does not signal another unusually cold, snowy winter, although it looks like might get one anyway!

So why are the eagles in Decorah and Fort St. Vrain working on their nests? Among non-tropical birds, many activities (reproduction, molt, migration) are linked to daylight length. We've primarily discussed the photosensitive period of bird life here, when daylight lengthens, gonads start swelling, and human watchers start counting the days until eggs are laid. However, birds also have a photorefractory period that in many northern birds begins slightly prior to summer solstice in mid-June. Gonads start shrinking, a new hormonal regime takes precedence, and many northern birds start preparing for winter. This might include social behaviors like flocking (something many mid-westerners start seeing in August), the cessation of care for fledglings, serious eating in preparation for migration, and dispersal from home territories. Researchers are quite sure that gonadal changes drive vernal (spring) behavior, but autumnal behavior is a lot harder to figure out.

Since daylight length is so important, we should probably chart it, right? We'll start with Decorah's photo-period from December 21, 2013 to December 20, 2014. At Decorah's latitude, the cycle of 365 days plots into a nice bell curve that looks something like this.

Daylight length in Decorah 2014. Charts here: http://goo.gl/71yGBx

Of course, there are other ways to plot daylight length. What happens if we consider each month separately? That graph yields a more complex pattern. Some months are rising, some months are falling, and daylight length is approximately equivalent on several days in spring and fall. For example, Decorah enjoyed 10:49 hours of daylight on October 20...very similar to February 20, with 10:44 hours of daylight. We'll get back to that soon.

Daylight plot by month.

Let's break it down even further. Figures one and two plot daylight length. But what does it look like if we plot the rate of change instead?

Do change rates also influence photosensitive and photorefractory periods? 

This graph shows the rate of change compared to overall daylight length across one year. The flat planes show time periods where daylight length is changing at a steady rate: +3 minutes per day at the top of peak one, and -3 minutes per day at the top of peak two. The valleys display transient states where daylight length is changing more rapidly, with the exception of the zero plane that occurs around each solstice.

For many birds in northern latitudes, the photosensitive period occurs primarily in cycle one and the photorefractory period occurs primarily in cycle two (with a little overlap into the tail end of cycle one). So why are they working on their nest during the photorefractory period, especially if weather prognostication isn't involved? A few thoughts.

• The eagles might be influenced by daylight length. Daylight length is quite similar in mid-February and mid-October. This might encourage bonding activities in the photorefractory period that echo those in the photosensitive period, especially between territorial mates. 
• The eagles might be impacted by Zugunruhe (migratory restlessness, which is also influenced by daylight length). I know Mom and Dad don't migrate, but many eagles do. Perhaps the suite of behaviors and hormones that direct migration in some birds influences our eagles to return to nest-building. Both adults certainly seem to have nest-building restlessness! 
• The eagles might be influenced by memory. Yes, light is the most important zeitgeber in Mom and Dad's world. However, study after study has shown that birds learn, remember, and draw on experience. Could last year's brutal winter be influencing this fall's flurry of nest preparation?

Light might be the most important zeitgeber in Mom and Dad's world, but it isn't the only one. In addition to dwindling hours of daylight, a flat-lined change rate, and no real photosensitive period day length matches, our eagles will be coping with cold and snow. As happened last year, we expect to see less of them until their gonads fire back up following winter solstice. Stay safe, Mom and Dad! We look forward to seeing you whenever you visit!

---

I got curious about daylight length in NE Florida since the eagles there just laid egg number one. How long are their days right now as compared to Decorah? The US Naval Observatory tells me they had ten hours and 33 minutes of daylight today, as compared to nine hours and 36 minutes in Decorah. Interestingly enough, our eagles lay eggs when we have roughly ten hours and 44 minutes of daylight. While eagles are influenced by other zeitgebers such as health, food availability, and age, this underscores the important role that photoperiod plays in the lives of our birds.

It's pretty straightforward to calculate photoperiod for the Decorah eagles, since they don't migrate and live at a latitude with obvious photoperiod changes. But how about connecting photoperiod to physical changes in the Arctic Tern, a bird that annually migrates from Greenland to Antarctica? Or the Kiwi, a nocturnal bird? How about birds that live in places like American Samoa, which has a shortest day only 1 hour and 41 minutes shorter than its longest day? The more I watch birds, the more questions I have about them. 

---

Things that helped me learn about this topic:

• Personal communication, Bob Anderson. Thanks for the lesson on photoperiodism! 
• Astronomical Applications Department of the U.S. Naval Observatory: http://aa.usno.navy.mil/index.php. If you like charting, this site is worth a visit.
• Paper: Circannual rhythms in birds, Eberhard Gwinner. Current Opinion in Neurobiology 2003, 13:770–778.
• Paper: Gwinner E, Scheuerlein A: Photoperiodic responsiveness of equatorial and temperate-zone stonechats. Condor 1999, 101:347-359.
• Paper: Dawson A, King VM, Bentley GE, Ball GF: Photoperiodic control of seasonality in birds. J Biol Rhythms 2001, 16:365-380.
• Book: Living on the Wind: Across the Hemisphere With Migratory Birds by Scott Weidensaul. This would make a great present for the bird-lover in your life! 
• Book: Sync: How Order Emerges From Chaos In the Universe, Nature, and Daily Life by Steven Strogatz. Another great book, although it isn't about birds!  
• Website: The Arctic Tern Migration Project. http://www.arctictern.info/ 

Thanks for reading! 

When will Mom and Dad...

When will Mom and Dad work on the nest? When will Mom and Dad mate? Are the eagles in Florida going to lay eggs early? "What's going on?!" everyone seems to be wondering.

Mom and Dad are on Eagle Time, which makes their behavior a little harder to pin down by our own clock! Still, their past behavior provides some clues. We should see work on the nest beginning in October and accelerating through the end of the year. In January, the eagles should move deeper into courtship. In addition to nest-building, we should see pair perching, vocalizations, bill stroking and pecking, and some footing or body stroking. Towards mid-to-late January, the eagles should begin bringing soft nesting material such as grasses, corn husks, and corn stalks in to line the nest bowl. Dad will begin sharing food (sometimes reluctantly), and physical contact will accelerate with even more touching, footing, and beak kissing. Mating will most likely begin in late January or early February, followed by egg-laying in late February or early March,

The US Fish and Wildlife Service's Bald Eagle Management Plan has a chronology for nesting activities in various parts of the United States.

Northern US. Includes Decorah, Fort St. Vrain, MN Bound

Southeastern US. Includes Florida Eagle cams, Berry College

As the chart clearly shows, the bald eagles in the Southeastern US are running on SEET (Southeastern Eagle Time) when compared to bald eagles in the northern US. However, eagles in both regions are right on time for their behaviors. In Iowa, Colorado, and along the Mississippi river in Eagle Valley, eagles are beginning to visit and work on nests.

• In Decorah, Mom and Dad are visiting the nest. A video from October 15th by Chico70 http://youtu.be/7RtD5VLwelU or watch live http://www.ustream.tv/decoraheagles
• The eagles in Eagle Valley haven't been seen in the nest lately, but they can sometimes be heard vocalizing. This area is a hot spot for migrating and overwintering eagles, and it's a wonderful place to hear birds even when they aren't on camera: http://www.ustream.tv/channel/eaglevalley
• In Fort St. Vrain, Mom and Dad have both been seen visiting the nest. A video from September 29 by eaglewhisperer18 http://youtu.be/DQkTcmPq0Hk or watch live http://www.farmyou.com/falcon_cams/eagle.html

If you can't keep up with the nests live, you are welcome to visit our Facebook page and/or forum.

---

The US Fish and Wildlife Service website contains some very good information on BE nationwide. Go to http://www.fws.gov/midwest/eagle/conservation/baea_nhstry_snstvty.html for a live web site or http://www.fws.gov/midwest/eagle/pdf/NationalBaldEagleManagementGuidelines.pdf for a .pdf of the management guidelines.