Friday, October 26, 2012

The Decorah Eagles Have Surprised Us With A New Nest!

Mom and Dad are surprising us again.  We have noticed in recent weeks that they seem to be building an alternate nest in the neighborhood.  This is not unusual for bald eagles, but it is new for Decorah.

The two bald eagle nests in the photograph above are located roughly 500 feet away from one of Minnesota's busiest interstate highways. The nest at the left was the first nest built by the resident bald eagles, who raised at least eight clutches before deciding to build a new nest in the same tree. I watched them raise young in the lower nest last year, observing in fits and starts as I drove by or sat immobilized in traffic. But I wasn't able to watch them build their new alternate nest, located above and to the right of the original nest, so I'm not sure exactly when they started.  I've seen eagles sitting in and perching by the alternate nest this fall, and I'm pretty sure the female will lay eggs in it come late winter.

Bald eagles build the largest nests of any North  American bird (Stalmaster 1987). So why would they invest the time and energy to build a new nest when the old one remains perfectly serviceable as far as we can tell?  Several hypotheses have been proposed to explain it:
  • Alternate nests may serve as 'insurance'. If the occupied nest is destroyed or rendered unusable, the eagles can quickly occupy the alternate without having to build a new nest.
  • Changing nests from year to year helps the eagles avoid parasites and 'nest mates' who are attracted by the food the eagles bring in. Shifting nests from year to year may help the eagles avoid parasitism.
  • The additional nests may be a way of marking territory. They warn other breeding pairs away and demonstrate fitness on the part of resident adults.
  • Eagles like to build nests.
Although we haven't seen it in Decorah before, multiple nest building is a relatively common activity: in most but not all instances bald eagles will have more than one nest in their breeding territory. In one study of 924 territories, eagles were found to have an average of 1.5 nests in their breeding area. In another study of 318 territories, 45% of eagles had two nests or more (Stalmaster, 1987). Eagle nests can be classified as active (shows or showed evidence of breeding by bald eagles during the current or most recent nesting season), alternate (intact or partially intact and used by bald eagles at any time during the past five nesting seasons, but was not used during the current or most recent nesting season), or abandoned (inactive through six or more consecutive nesting seasons). It takes six years for a nest to go from 'alternate' to 'abandoned', since last year's alternate nest might become this year's occupied nest. Why? Only the eagles know for sure.
It is interesting to us that, like the bald eagles in Lino Lakes, the Decorah eagles occupied a single nest for several years before beginning an  alternate. The new nest in their territory is located nearer the trout hatchery, roughly 300-500 feet from their current nest near the woodshed. We don't know at this point which nest they will occupy next spring. Bob did find a dead buck beneath the foot of the 'alternate' tree, which could have played a role in their decision to choose this particular location. But we don't know that for sure.
Here's a peek at the alternate nest (photo by Jim Womeldorf):

To recap:
  • Bald eagles commonly build multiple nests on their nesting territory.
  • Mom and Dad have started construction of an alternate nest about 300-500 feet from their current location.
  • We will not do anything that might influence them to abandon construction of this nest, including camera installation or nest invasion. We cannot and will not disturb them: building multiple nests is part of eagle life. The health and welfare of the eagle family is more important than the nest they choose or our desire to watch them on camera. We'll miss following them, but it is exciting to see them building a new nest! Once again, Mom and Dad are giving us new insights into the lives of bald eagles.

If the eagles do adopt the new nest:
  • We will provide photographs and updates from the ground.
  • We will consider putting a camera in it next year, in early fall.
A few things to ponder: 
  • Is the length of time a mature pair stays on territory correlated with the construction of alternate nests? Five or six years would be plenty of time for parasites and 'nest mates' to become problematic, or at least annoying.
  • Is the length of the bond between the pair a factor? More experienced (older) Bald eagles seem to be better at parenting in many ways. Would a more experienced pair be more likely to build an alternate nest for insurance?
  • Does the actual or perceived encroachment of other adult eagles play a role? If other eagles are present, resident eagles might feel a little more compelled to mark territory.
We would really like Mom and Dad to use the nest they have occupied for so long, but we cannot and will not interfere if they decide to use the new nest. As we said in an earlier post on intervention, their lives are a gift we have been privileged to share. We can only hope we'll get another chance in 2013.
Stay tuned and we'll keep everyone updated as the situation develops!

Things that helped me write and learn about this:
  • Book: The Bald Eagle. Mark Stalmaster, 1987. This is a wonderful book, and the eagle illustrations are charming.
  • Book: The Bald Eagle: Gerrard and Bortolotti. Another great book.
  • Green Value Nursery, Hugo, MN. Thanks for letting me park and walk across your land to get photos of the Lino nest. I'll buy flowers from you next spring!
Note: I got an email from Nancie Klebba in Lino Lakes. She says:

I spoke with a few people on the board that live on the lake and the City's environmental coordinator and they believe the nest has been on Peltier since about 1993 or 1994.

Marty the environmental coordinator said he would check back to see if he could find the exact date.  He  also said the old nest has not been used the last few years.

Thanks, Nancy!

Tuesday, October 16, 2012


At whatever moment you read these words, day or night, there are birds aloft in the skies of the Western Hemisphere, migrating. If it is spring or fall, the great pivot points of the year, then the continents are swarming with billions of traveling birds...
- Living on the Wind: Across the Hemisphere with Migratory Birds

We get a lot of questions about migration. Do the Decorah eagles migrate? Do our Peregrine falcons migrate? Where do they go when they leave? We fitted two young eagles with tracking devices in part to help answer some of these questions.

I used to think migration was very simple. Like a lot of people, I thought that all birds except chickadees, pigeons, crows, and woodpeckers migrated once it got cold. They went south to escape snow and ice, returning to nest when the weather warmed up. 'South' was anywhere it didn't snow, or at least didn't snow much: Georgia, the Gulf Coast, South America. I had no idea that many birds don't migrate, or that 'south' could be Minnesota or Wisconsin. Peregrines and Bald eagles taught me otherwise.

Bald eagles and Peregrine falcons are partial migrators - that is, some members of the species migrate and others do not. Mom and Dad Decorah both stay put on their territory year round. They have abundant food and water, so there is no reason to leave. Belinda, the falcon at the Xcel Allen S. King plant in Stillwater, Minnesota, also has a history of staying put. There is abundant food on territory and an open water supply slightly south of the plant. In both cases, the resident bird or birds' presence may make it easier for them to preserve their territory without having to chase off interlopers that set up house in their absence.

However, other birds, even those with defined nesting territories, leave. Immature and adult Bald eagles congregate in large numbers by open water along the Mississippi river, a very important flyway for many kinds of birds here in the midwest. Belinda may stay put but it is not uncommon to see interloping falcons at empty nestboxes: just two weeks ago, Brenda Geisler spotted a Peregrine falcon from North Dakota at the Great River Energy nestbox in Elk River, Minnesota. What is migration and why do some birds stay put while others leave?

Among birds, migration is the regular, endogenously controlled, seasonal movement of birds between breeding and non-breeding areas (Salewski and Bruderer 2007). Bald eagles and Peregrine falcons are partial migrators - that is, some members of the species migrate and others don't. This is the most common type of migration, which makes sense since migration is driven by a number of factors, including daylight length, food availability, weather, the time it takes to raise young, and the distance between wintering and breeding grounds. Migration allows exploitation of different habitats as environments change seasonally or successionally (Dingle, 1996). Food availability seems to play a very important role in the migration of Bald eagles: inland northern Bald eagles tend to move southward after ice and snow start putting a lid over their favorite food source - fish, while southern Bald eagles are thought to move northward once warm weather drives fish into deeper water (there is some debate about this). Weather can also impact migration timing in other ways: for example, a favorable wind pattern might help compel a bird to leave for its wintering or summering grounds if other factors are in place.

In general, young Bald eagles are much more nomadic than adult Bald eagles, something we've seen with both D1 and D14. In spring and summer, hatch-year and young sub-adult eagles may fly north to over-summering areas in the northern United States and southern Canada, returning to their birthplace in late summer or early autumn. Or they may not, as we've seen with D14. We don't know why D1 dispersed so much farther than D14 in her first year. Among Peregrine falcons, females tend to disperse farther than males* , but I haven't been able to find documentation of gender-related dispersal in Bald eagles. I'll keep looking, and you are welcome to post links and resources in the comments. Perhaps further study will shed some light on the issue. Eagle migration and dispersal is very complex.

So how do birds navigate? Migration studies have found four major methods:
  • Magnetic sensing: Some birds, including pigeons, are able to use the direction and strength of Earth's magnetic fields to orient themselves.
  • Geographic mapping: When I'm in Minneapolis, I use a number of tall buildings to help me orient the city. It turns out that birds do the same thing, using landforms and geographic features such as rivers, coastlines, and mountain ranges to guide them.
  • Celestial navigation: migratory birds use the position of the sun (during the day) or the rotation of stars (at night) to orient themselves. Experiments done by Dr. Emlen in 1967 indicate that celestial navigation is learned.
  • Learned Routes: Some bird species, such as sandhill cranes and snow geese, learn migration routes from their parents and other adult birds in the flock. Once learned, younger birds can travel the route successfully themselves.
What do Bald eagles and Peregrines do? It seems they most likely use multiple sources of information, including celestial and magnetic clues, light polarization, wind patterns and direction, and geographic cues (which would likely be highly correlated with geography). Although parents and young don't migrate together, D1 and D14 were in the company of other eagles every time we saw them, so I suspect that some degree of learning, or at least following, also plays a role. Again, we really have no idea why D1 and D14 behaved so differently.

We suspect that D1 will come back to NE Iowa this winter, following Lake Superior's western shore. We don't know whether she will come down along the Mississippi river or go back into western Wisconsin, where she spent so much time in the fall of 2011. But we are looking forward to finding out!

Some things that helped me write this post:
* Regarding male Peregrine dispersal: of course, there are exceptions. We released Zeus, the male at Woodman Tower in Nebraska, in Rochester New York. We were very surprised when he showed up in Nebraska. Did he fly west intentionally or was he some how blown off course or lost?