Tuesday, November 08, 2016

Where did all of those eagles come from?

Decorah North watchers know that we've seen an incredible number of eagles in around the nest, especially November 4th through November 7th. The tally looks something like this:
  • Friday evening, November 4th: What appear to be two sub-adult eagles are seen in the vicinity of the nest. A lot of vocalization can be heard off-screen, including chattering and pealing. Video: https://youtu.be/jLf2gYPVcyk
  • Saturday, November 5th: At about 8am, an intruding adult is in the nest with a sub-adult perched nearby. Mom and Dad North chase them off. At 6:03pm, a juvenile bald eagle perches on a branch near the nest but is chased away by Mom North. Chattering and pealing is heard offscreen several times throughout the day. Video: https://youtu.be/INVx8RInv98
  • Sunday, November 6th: A juvenile eagle is spotted on a lower right branch near the nest at 9:00am, but the true follies don't get started until 1:30pm, when an intruding adult female flies into the nest with two sub-adults (or an juvenile and a sub-adult) in hot pursuit. There may have been a third sub-adult in a tree near the nest, and there was definitely another sub-adult in a tree in the pasture. The immature and adult intruders are all driven out, but another adult (or the same one - we don't know) shows up later with food and is driven away. A lot of pealing and chatter can be heard off screen throughout the day. Video: https://youtu.be/zabzKoQjQnY
  • Monday, November 7th: A sub-adult eagle visits the North nest to play house at about 7:30am: https://youtu.be/Xaw5Yb_FUkY, followed by a pool party later in the day, when five immature eagles, one adult eagle, and two sandhill cranes were counted in a stretch of the stream that flows to the east of the North nest: https://youtu.be/hbhMkNcn7Y4
We are getting asked why there are so many intruding eagles here as compared to Decorah, and why so many eagles (and cranes!) right now. While the North Nest has tended to have more intruders, we haven't seen anything quite like this before. A few ideas:

On the question of location (North versus Decorah)
  • The North Nest is in a wilder area with more forests and natural terrain. While Decorah is very beautiful, it is a city on the eastern edge of an extensive agricultural area that stretches across the entire state. Decorah and areas west have much less habitat for wild animals. While only one of our tracked eagles have been seen at the North nest, many of them have visited the North area.
  • Consider an eagle migrating from Ontario to winter along the Mississippi. It will most likely fly along the west shore of Lake Superior before it peels off the tip where Minnesota and Wisconsin meet. At that point, it is about a 150-mile flight almost straight south to Lake Pepin - something eagles can do easily in a single day, especially if the winds are favorable. If they aren't, Wisconsin's lake country offers plenty of habitat for rest and relaxation. Once our eagle gets to Lake Pepin, it might chose to stay - many eagles do - or it might go further south to, say, Eagle Valley. The shorter straight-line path does not follow the river but cuts in-country through the vicinity of the North Nest, narrowly missing Decorah.
  • The North eagles are living just inside a north/south wind-funnel of sorts that cuts through the hills and channels wind - and birds - for miles. Topography influences migration in many birds, including bald and golden eagles. Looking at the terrain, it is no surprise at all to see eagles congregating here - even if the Norths don't exactly put out the welcome mat! 
In short, the North eagles are living in a region that has more habitat for bald eagles and other animals, is in a relatively straight line between critical upper and lower stretches of the river, and is in a long north/south valley that funnels wind and aids migration.

On the question of timing (why now)?
I asked bald eagle expert and board member Brett Mandernack if the eagles he is studying had left their summer grounds. He replied that they had, although given the warm weather, many were staging in Wisconsin and along the Mississippi river north of Eagle Valley. Presumably, these are eagles on their way south to somewhere else, but in no hurry given the weather. 

Unlike many birds, bald eagles migrate almost exclusively during the day. They rely heavily on thermal soaring to aid their flight, so a warm, sunny day with favorable winds can send eagles soaring by the thousands.


Let's look at the wind maps from the week of November 4, 2016. Wednesday through Friday would presumably have been excellent migration days: especially Thursday, which had favorable winds in addition to wonderful weather! But the winds started to change on Friday and were unfavorable from Saturday afternoon through at least late Monday afternoon. Why would eagles go anywhere if they had access to food, water, and high-quality habitat? The area around the North Nest provided a perfect staging spot for migrating eagles headed south. Perhaps large groups of eagles could have been found in many valleys in NE Iowa this weekend. We were fortunate to get to see these!

Chasing an intruding juvenile eagle from the North Nest

Chasing an intruding adult eagle from the North Nest

A visiting - and playful! - sub-adult eagle

Immature and adult eagles at the river east of the nest

Bathing - and interacting - at the river's edge

Sandhill cranes and an adult bald eagle
Chasing an intruding adult eagle from the North Nest

Did you know?
  • Juvenile eagles are fledglings in their first year. They are a darker brown than sub-adult eagles, with less mixed white and lighter brown plumage. Sub-adult eagles are 2-4 years old. They have very mixed plumage compared with juveniles and adults. Adults get their white heads and tails at around five years of age. Immature can refer to juvenile and sub-adult eagles. For more on aging eagles by plumage, visit this website: http://bit.ly/1aGakDV
  • Although bald eagles take advantage of solar power by migrating during the day, many birds migrate at night. It is believed that night migrations allow them to avoid heavy winds and storms, since nights are often calmer; keep away from predators, especially birds of prey that hunt and migrate during the day; and preserve water and energy, since cooler night-time temperatures will induce less heat-related panting.
  • Check out Brett Mandernack's .pdf of D1's migration paths. You can see a real preference for direct lines, especially on repeat journeys. She tended to cut down pretty directly through WI, unsurprisingly passing through the North Nest region, although she was never at the nest or its vicinity that we know. 

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