D27 sent another postcard – from Decorah! Our little eaglet began her fall 2019 migration on August 11. She arrived on her winter territory on August 25, 14 days and about 690 miles later.
Between the 11th and the 13th, D27 flew an incredible 336 miles in 39 hours, averaging a speed of almost nine miles per hour. Her stats look like this:
- She flew her biggest day on August 12, traveling roughly 138 miles, or almost 20% of her total journey.
- Her earliest day began at 5:06 AM on August 13. She clocked in at a little over 136 miles: her second-biggest day!
- August 13 was also the day she flew her fastest, longest single trip. D27 traveled 74.9 miles in three hours, averaging almost 25 miles an hour.
Daylight length in northern Ontario
Back during the summer solstice on Friday, June 19, D27 did not experience true night. During the darkest period of her day, which lasted three hours, the sky would have been dark blue and a faint glow would have illuminated her horizon. True day lasted for a little over 16 hours.
Astronomical twilight at Beaudry Provincial Park, Manitoba, Canada. Photo credit Workie.
By the time D27 left for Decorah, the length of her day had fallen to a little over 14 hours, and night – something that didn’t happen at all less than two months ago – had grown to four hours and 25 minutes long.
Mated territorial adult bald eagles at this latitude often leave their breeding grounds between mid-October and early November. But D27, an unmated sub-adult, is free to go wherever her wings take her. Brett’s study gives us a nice look at how daylight length impacts migration timing – and how timing is mediated by other factors, including mates, territory, and weather.
Speaking of weather…
timeanddate.com archives weather for a station not far from D27’s summer grounds in northwestern Ontario. August 12 and 13 were excellent days for migration! Winds were light and consistently from the north, helping D27 minimize powered flight and maximize energy as she made her way south to Decorah.
Most birds of prey are diurnal migrators, but many otherwise diurnal birds migrate at night. This might help them conserve energy, keep body temperatures low, and even catch a few Zzzzzz’s on the wing. If you are interested in being out during a big nocturnal migration – or spotting migrants during the day – check out http://birdcast.info/. As a general rule of thumb, September nights with favorable winds tend to bring in birds, especially if the moon is at or near full.
How does D27’s fall migration this year compare to last year?
2018 and 2019’s were a little different. In 2018, D27 left NW Ontario on August 9. She traveled very quickly through Canada but detoured to western Minnesota and east central Iowa before returning to the Decorah area on about October 18.
Why do the two years look so different? Like D1, D27’s fall migration efficiency improved from the first to the second year. Brett stated that experience at identifying landmarks, remembering successful foraging
areas, or taking better advantage of weather conditions might contribute to increased efficiency. Juvenile eagles also seem a little more apt to wander. Their curiosity and willingness to take risks that older, more experienced eagles avoid might be one way in which eagle populations expand their range.
We’re hoping to get a chance to look for D27 in the weeks to come! A million thanks to Brett Mandernack and the Eagle Valley crew for sharing their information, expertise, and data with us! If you’d like to read Brett’s Decorah bald eagle tracking paper, follow this link: https://www.raptorresource.org/pdf/Satellite-Tracking-Decorah-article-text-2019-04-01-with-abstract.pdf. If you’d like to explore the travels of the Decorah eagles Brett has tracked, check out our interactive maps: https://www.raptorresource.org/learning-tools/eagle-map/. Fly high, D27! We hope to see you – at a distance – in September!