Thursday, January 22, 2015

Owls in N2

Owl Defense @ roughly 12:00AM
On Monday morning (01/19/15) at about 12:07AM, our cameras caught an altercation between an invading great horned owl and a defending bald eagle parent. While we've seen brief nest invasions before, we've never seen owls that appear determined to take over the nest. We've had a lot of questions and I'll attempt to answer the most commonly asked here.

Don't Owls Nest In Cavities?
Owls may nest on ledges, dense brush, cavities in trees, holes in the ground, and nests built by other birds. Unlike many birds, owls don't usually make their own nests. Cavity-nesting owls can't drill holes in trees, so they utilize holes made by woodpeckers, rot, or broken branches. While burrowing owls can dig, they prefer to take holes excavated by prairie dogs, skunks, armadillos, or tortoises. Owls that prefer stick nests take nests made by other birds instead of building their own.

Our ideas about owls and cavity-nesting are probably influenced by the 20th-century decline of large nest-building raptors like bald eagles and red-tailed hawks. With fewer bald eagle and red-tailed hawk nests to steal, great horned owls might have been more likely to use cavities. But with bald eagle and red-tailed hawk populations on the rise, there are more nests and great horned owls might be back to taking them.

When is it adopting and when is it stealing? According to much of what I've been reading on the subject, great horned owls often adopt red-tail hawk nests. Since red-tailed hawks aren't using their nests in January and February, it doesn't seem much like stealing to me. Unfortunately, bald eagles and great horned owls both lay eggs at roughly the same time. When Mom and Dad are trying to use the nest they built, our feelings change. How dare those owls steal another bird's nest!

Interestingly, a lot of what I read talked about red-tailed hawks using nests for just one year. While RTH don't always build new nests, do those that do build a new nest every year because they are behaviorally programmed to do so (as was suggested in much of the literature), or do nest adopters help drive their behavior? Great horned owls nest earlier than red-tailed hawks, but their seasons overlap just enough to require replacement nest building on the part of the red-tailed hawks.

Why Aren't The Owls At N1?
We remember N1 as a large bowl surrounded by sticks and filled with grass and cornhusks. But Mom and Dad aren't replenishing it any more. As recent camera work has shown us, the surface is cold and bare and the nest, which is compacting under its own weight, has flattened. While we don't know for sure, I suspect the owls find the soft grasses and bowl-like shape of N2 much more inviting than the barren exterior of N1.

Could The Owls Take Over N2?
I can see three possible scenarios for the nest.

In scenario one, the owls begin laying eggs in N2 at night while the eagles are roosting elsewhere. When the eagles come to the nest the following morning, they find it defended by a pair of owls. In scenario two, the owls keep tangling with the eagles and the eagles abandon the nest. I don't know whether they would reclaim N1, try to build a new nest, or attempt to take over an existing nest somewhere else. In scenario three, as the eagles begin to roost more in the N2 tree at night, it's possible their presence would be a deterrent  and/or the eagles would continue to try and drive them away.  All scenarios are speculation and we will just have to wait and watch nature take its course.

How likely is a nest takeover? Great horned owls are about a third the size of bald eagles, but they are formidable opponents. Unfortunately, I couldn't find much data on nest takeovers, and anecdotal evidence wasn't helpful in figuring out who would probably win. I'd bet on whoever lays eggs first, but even nesting birds have been driven away by nest invaders.

Would The Owls and Eagles Be A Threat To One Another's Young?
No one has appeared especially interested in N1 yet, but suppose we ended up with owls in one nest and eagles in another? We've seen owl predation at Eagle Valley and while Mom and Dad have food preferences, they appear to eat almost anything that flies, swims, or walks on four legs. Both species are territorial and aggressively defend their nests, so it's hard to believe they wouldn't attack one another. Having said that, nest attacks are hazardous and there are accounts of competitive species living side by side. A study of three different species of owls in Finland revealed a surprisingly complicated set of interactions, with prey competition playing a larger role in reduced production than direct aggression. A field report written in 1908 reported great horned owls and bald eagles nesting in the same nest. The author wrote:

This nest had been used for years by Bald Eagles, each annual addition of materials increasing the height until it reached fifteen feet up between the main forks of the tree. At the bottom it was eight feet through, where it was jammed in between the forks, and from here it tapered to four feet, ten feet up, and again spread out to six feet on top where the two Eagle eggs rested on soft, dry grass in the rudely-formed depression. Four feet from the bottom of this huge pile of branches and debris was the Owl's home, containing two freshly laid eggs.

So in short, I don't know. Bald eagles and great horned owls overlap in a lot of places, they don't compete for exactly the same food base, they forage at different times of the day, and the vicinity is well-stocked with prey. We'll be watching intently (and asking for your help with documentation) if they end up nesting in close proximity. We will continue to watch the nest or nests regardless of who ends up nesting in them.

Can You Do Anything About It? 
No. This is a natural event and we believe it would be unethical to interfere. Beyond that, our permits don't allow us to take healthy owls and interference could easily do more harm than good. If an eagle or owl is hurt, we will do our best to find it and get it to a rehabilitation facility.

Will The Owls Take Advantage Of The Hatchery?What do great horned owls eat? Like eagles, almost everything. Cornell University tells us that they eat mostly mammals and birds—especially rabbits, hares, mice, and American Coots, but also many other species including voles, moles, shrews, rats, gophers, chipmunks, squirrels, woodchucks, marmots, prairie dogs, bats, skunks, house cats, porcupines, ducks, loons, mergansers, grebes, rails, owls, hawks, crows, ravens, doves, and starlings. They supplement their diet with reptiles, insects, fish, invertebrates, and sometimes carrion.

Having said that, they have a real preference for mammals. I don't think they would take advantage of the hatchery unless mammals were in very short supply.

What Do You Think About It?
We would like the eagles to stay in the nest and be productive. If that doesn't happen, we would like the nest takeover to occur without injury or loss of life to the eagles or the owls. More than anything, we hope that they don't engage in a prolonged inter-species conflict that results in the maiming or death of young. We won't interfere with life in the nest but as I stated earlier, if an eagle or owl is hurt, we will do our best to find them and get them to a rehabilitation facility. Remember that while both species have serious talons and beaks, conflicts don't have to involve injury.

A video chronology of owls at N2. Please let me know if the comments if I'm missing one.

10/16/14: Noisy GHOs on N2:
10/26/14: GHOs in N2:
11/18/14: GHOs are back:
12/17/14: Early morning visit by the owls:
01/02/15: The owls are back:
01/04/15: The owl are back (again):
01/05/15: Two owls visit:
01/07/15: An owl came to check out the nest:
01/10/15: Owl on nest:
01/18/15: Owl strike on nest:
01/20/15: Owl is back:
01/22/15: Owl with mouse gift for his mate:

I have written a couple of posts on owls in the last few years. A few links:

Things that helped me learn and write about this subject: 

Competitive and Predatory Interactions among Raptors: An Observational and Experimental
Study. Author(s): Harri Hakkarainen and Erkki KorpimäkiSource: Ecology, Vol. 77, No. 4 (Jun., 1996), pp. 1134-1142. Published by: Ecological Society of America. Stable URL: 

Bald Eagle (Haliætus leucocephalus) and Great Horned Owl (Bubo virginianus) Occupying the
Same Nest. Author(s): J. Warren JacobsSource: The Wilson Bulletin, Vol. 20, No. 2 (Jun., 1908), pp. 103-104. Published by: Wilson Ornithological Society. Stable URL: 

Close Nesting and Aggression Contacts between Great Horned Owls and Red-Tailed Hawks
Author(s): Dwight G. Smith. Source: The Auk, Vol. 87, No. 1 (Jan., 1970), pp. 170-171. Published by: American Ornithologists' Union. Stable URL: 

Many thanks to Karla Bloem from the International Owl Center for providing information and feedback. You should visit the website (seriously, please do.):

Tuesday, December 02, 2014


We have a day for giving thanks. We have two for getting deals. Now, we have #GivingTuesday, Tuesday, December 2nd, a global day dedicated to giving back! If you like the work we do, please consider making a donation. You can donate online via paypal or mail a check to the Raptor Resource Project, PO Box 16, Decorah, IA 52101.

What does the Raptor Resource Project do? We:

  • Work with government agencies, private businesses, and other non-profits to monitor and band peregrine falcons at over 40 sites. Our region stretches for well over 500 miles and encompasses parts of three major rivers - the Minnesota, the St. Croix, and the Mississippi. 
  • Provide one of the world’s largest public wildlife education programs to countless school kids through our live online streams, interactive chat, facebook page, and blog. Our Decorah eaglecam alone has been watched by over 302 million people worldwide since 2011. We maintain over 20 public and private cameras to monitor and learn about birds of prey.
  • Build and install nest boxes to bolster bird of prey populations in Iowa, Wisconsin, and Minnesota. 
  • Provide information and consultation free of charge to other organizations and businesses who are interested in initiating their own nest box programs.
  • Track and share data about free-living bald eagles in conjunction with Brett Mandernack at Eagle Valley.

These things all take money. As of 2014, our annual expenses were hovering around $87,000 per year:

  • Staff costs a little over $42,500 annually.
  • Equipment – computers, camera upgrades and maintenance, tools, encoders, software, transmitters, and so on – costs around $17,000 annually.
  • Supplies – primarily cable, tools, climbing equipment, banding equipment, bands, installation hardware, maintenance equipment, and lumber – cost around $5,000 annually.
  • Satellite data costs $4,225 dollars annually.
  • Internet access costs $4,500 annually.
  • Other/Miscellaneous costs around $9,000 annually. This category includes gasoline, electricity, travel-related costs, equipment fabrication, and a furnace and propane so we can heat the shed!
  • One-time costs and special projects vary widely. In 2013, for example, we spent $19,000 in Decorah to get video cable over to N2. While that was unusually expensive, it isn't uncommon for costs to crop up given the amount of cams and equipment we have. 

We have some exciting projects coming up in 2015 and 2016, including a wild Philippine eagle cam and an expansion of our eaglecam project! Our income is generated entirely by small donations from viewers of our various cams, and we sincerely appreciate your generosity and support of the Raptor Resource Project mission. Would you please help us make a difference with your donation?

Thank you so much for your support and we hope you enjoy watching in 2015!

Wednesday, November 19, 2014

Why Are The Eagles Working On The Nest?

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:
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 seconds per day at the top of peak one, and -3 seconds 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. Yes, this is purest WAS. 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: 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. 
Thanks for reading! 

Friday, October 31, 2014

Eyes in The Night!

In honor of all things spooky, we've been running a feature called 'Eyes In The Night' over on our facebook page. So far, we've seen raccoon, deer, opossum, bobcat, hare, great horned owls, skunk, great crowned night herons, and bullfrogs. We've watched them forage, hunt, eat, and very occasionally interact with one another under IR light, which they can't see. It's fascinating to see just how many animals are busy after dark!

Animals can be classified as nocturnal (primarily active at night), diurnal (primarily active during the day), cathemeral (sporadically active during the day or night), and crepescular (primarily active at dawn and dusk). Whether nocturnal, cathemeral, or crepescular, animals that are active under low-light conditions may have bigger eyes and ears, darker colors, more distinct vocalizations, and quieter movements than their strictly diurnal counterparts.

Opossum at Eaglecrest 
Why do the animals and birds we watch after dark have such large glowing eyes? Because their eyes:
  • Tend to be larger and more protruding than those of diurnal or daytime animals. 
  • Often have pupils that open more widely than those of diurnal animals.
  • Often have slit rather than round pupils. Slit pupils are more efficient at opening and closing quickly.
  • Usually have more rods than cones. Rods work better in low light, detecting motion and basic visual information, while cones work better in bright light, registering detail and detecting color. Some nocturnal animals have no cones at all.
  • May have a special light gathering structure called a tapetum, which amplifies the amount of light that reaches the retina. The tapetum reflects light that has already passed through the retina back through the retina a second time, giving it another chance to strike the light-sensitive rods. The animals we watch can't see IR light, but their widely dilated eyes certainly reflect it!

Deer are crepuscular, or active at dawn and dusk.
Their large, moveable pinna give them excellent
directional hearing. 
Why do the animals we watch after dark spend so much time listening? Many nocturnal animals rely on their hearing as much or more than their vision. How are their ears different from those of diurnal animals? They might have:
  • Enlarged pinnae (the outer part of the ear), facial disks, or other features that help them gather and condense sound.
  • Directional location. Think of ears as a microphone. In some animals (bobcats, for example), pinnae can be moved to funnel sound into the inner ear, helping localize the source of sound. Other animals, like owls, have offset ears that provide a precise x/y prey point based on a sound’s location.
  • Inner ear structures that maximize high frequency sounds (like mice rustling in the leaves). This might include denser, shorter auditory hairs, and denser auditory nerves. All the better to hear you with, my dear! 
  • Ultrasonic hearing. Bats are famous for echolocation, but some moths and nocturnal butterflies have ultrasound-sensitive ears on their wings to help them escape. 

Opossum flehvening at Eaglecrest
Why do the animals we watch spend so much time smelling things? In a low to no-light world, scent conveys important information about potential mates, rivals, predators, and dinner. Nocturnal animals:
  • Often have a larger olfactory epithelium, which improves their sense of smell. The sense of smell is most highly developed in nocturnal mammals, although some nocturnal birds also have a sense of smell. An improved sense of smell helps nocturnal animals find food and avoid becoming dinner under low-light to no-light conditions.
  • Often communicate via scent marking and smell. Scents intimidate rivals, attract mates, convey information about health, and provide information about numbers and types of animals in any given place. 

Nocturnal animals are very sensitive to touch. Many of them have special adaptations to help maximize touch in low-light or no-light situations, including:
  • Vibrissae, or whiskers that serve as tactile organs. While many animals have vibrissae, they tend to be more prominent on nocturnal animals or animals that hunt in low light. Vibrissae may be found on an animal’s face, forelegs, or feet. 
  • Sensory maps that maximize the input from vibrissae, antennae, and other tactile sensors. A study by biologist Dennis O’Leary found that a mouse’s sensory map is dominated by clusters of neurons that process whisker signals. A cluster for one whisker is bigger that the cluster of neurons dedicated to the mouse’s entire foot. 
  • A higher percentage of the cerebral cortex dedicated to touch. For example, almost two-thirds of the area responsible for sensory perception in a raccoon's brain is specialized to interpret touch. 
In a dark world, special adaptations help animals find food and mates, avoid predators, and defend their territories. I am grateful to have a chance to see them, spooky or not! Every time I step outdoors after dark now, I think about all of the animals that might be active around me.

Did you know...
While we don’t understand all of the ways touch works, image maps called cortical homunculi help map tactility. Check it out:
Links that helped me understand this:

Wednesday, October 15, 2014

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.
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 for a live web site or for a .pdf of the management guidelines.

Monday, September 29, 2014

Four and Dispersal

Four back in Decorah. Photo credit Bob Anderson.
Last week, Four took a little trip 22 miles northwest of her natal nest, although she has since returned and reconnected with Mom and Dad. What was Four doing? Will she ever permanently leave Decorah? How do we define and classify the behavior of a highly nomadic species like the bald eagle? I can't tell you what Four is thinking, but I can tell you a little bit about eagles and their movements.

Dispersal is the movement of individuals from their point of origin (natal area) to where they reproduce or would have reproduced had they survived and mated. Navigation is an animal's movement around its territory or home range. Migration is the regular, seasonal movement of animals between breeding and non-breeding areas. While D1 hadn't yet bred, she was clearly migrating back and forth between a non-breeding territory (Polar Bear Park) and a territory in which she would be likely to breed (NE Iowa).

It is theorized that animals disperse for one of three reasons:
  • To avoid inbreeding
  • To avoid kin competition
  • To colonize new habitat
A detailed discussion of these theories can be found here, but all three dispersal strategies are linked to reproductive success. Inbreeding takes a toll on reproductive fitness in the long run, and kin competition can have immediate deleterious consequences, especially if kin are competing over limited resources. Colonizing new habitat disperses a population across a wider area, so local extinctions have a lower chance to kill every individual in a population.

Having said that, D1, D14, and Four didn't sit down and check off a list of reasons prior to dispersing. Dispersal seems to be triggered by a number of factors, including hormones, body condition, reduction of parental provisioning, increased locomoter activity, and external factors that include favorable winds and full moons for night flyers. In short, our eagles learned to fly and hunt, Mom and Dad decreased feeding, an adrenally-produced hormone called corticosterone soared, and the eagles dispersed. Reports of Four and the other eagles playing 'tag' with Mom and Dad shortly before dispersal may really have been a hungry, hormonally-charged young eagle chasing down its parents in an attempt to secure food from them. Suddenly tag seems a little less playful.

Unlike dispersal, migrating animals usually move from one geographic region to another without using intervening habitat.  But are dispersal and migratory behaviors mutually exclusive in an highly nomadic animal that usually doesn't breed until it is four years of age or older? What limited data we have indicates they aren't. D1's first year had characteristics of both dispersal and migration. She left her natal territory on August 13, 2011 and flew 262 miles northwest, arriving at her northernmost destination on September 6th. She spent the next four months slowly traveling south, with long layovers at Yellow Lake and Black River Falls. She arrived back in NE Iowa on December 24th, 2011.
D1 in 2011
D1 spent her winters navigating throughout NE Iowa and SE Minnesota, but she refined her north/south movements considerably. By 2014, her 808-mile trip north to Polar Bear Park took just 16 days. Although she wasn't yet breeding, her trips fulfilled the criteria for migration: they were seasonal, endogenously controlled, and didn't use much intervening habitat.

RRP board member Brett Mandernack is researching the migratory behavior of immature bald eagles. If there is a difference in immature vs. adult eagle migration, when does this change occur? Is it gradual over the first several years of the eagle’s life or is the change abrupt? D1, D14, and Four are part of his work. The paper from his first study, which examines data collected between 1999 and 2006, was published in the Journal of Raptor Research in 2012. An abstract can be found at

So will our eagles choose to nest in Decorah as adults? It's not impossible, but it isn't likely either. Dispersed young may return to the vicinity of the nest, but they don't appear to spend time there as sub-adults or adults. A study of 878 bald eagles found a median natal dispersal distance of 42 miles (69.2 km) overall, with females dispersing farther than males. Unlike human families, the young do not return for a visit with grandchildren in tow.

Eagle cluster map. Decorah is marked by the red 375, but note other favorite spots
Does an eagle's initial dispersal or overwintering perambuation tell us where it might nest as an adult? D1's favorite wintering area was near Elkader, Iowa, about 35 miles south of her natal nest - in line with average disperal distances. Unfortunately, her transmitter stopped talking to us. We'll try to get a visual on her this winter but we can't guarantee it.

As I mentioned at the beginning of this post, Four is back in Decorah and reunited with at least one parent, although we haven't seen them feeding her. We'll let everyone know if she leaves again.

Things that helped me learn about this topic:

Thursday, September 18, 2014

Flight Maps

Will Four ever disperse? We've been asked that question via facebook, email, and our forum. Only Four can answer that question, but I thought it might be worth looking back at D1 and D14 for clues.

D1's first flights, 8/01/11 to 8/12/11
The map above shows D1's first flights. She fledged in June, at 81 days of age, and we attached the transmitter to her on July 18, 2011. The maps shows her exploring her natal territory, traveling simple, short back and forth paths as she learns proficient flying, food-finding, and navigational skills.

D14's first flights, 8/01/12 to 8/12/12
D14 was 78 days old when he fledged in late June, He is less centered around the nest and hatchery than D1 was, but his initial exploratory flights are very similar. Like D1, he is flying short, simple back and forth paths as he gains skill and experience. 

Four's first flights, 8/01/14 to 8/12/14
Four's fledge occurred at approximately 76 days old, in late June. Like D1 and D14, her early flight pattern is relatively simple  - a lot of short back and forth explorations. Her first "loop" occurred on August 10 and was just 1 mile long (.5 miles out and back). 

So when does a hatch year eagle stop exploring and start dispersing? D1 was quite obvious. On August 13 2011, she flew north 14 miles toward the Iowa/Minnesota border. By September 6, she was 262 miles north of the nest. She turned south and came back to Decorah, arriving back in NE Iowa on December 24, 2011. Before leaving on August 13, she had never been recorded more than .98 miles from her natal nest.

D1's 2011 dispersal
The map above displays D1's entire dispersal. On August 13, she went from local travel along simple paths to long-distance travel along more complex paths. Her large loop included several smaller loops with fewer of the simple back and forth flights that we saw her first weeks on the wing.

D14's 2012 dispersal
Unlike D1, D14 spent more time flying around the Decorah area and left town twice: once on September 5, for a 24-mile flight south, and once on September 22, for good. So did he disperse on September 5 or September 22? My money is on September 22. D14's movements seem to indicate a higher degree of pre-dispersal restlessness, but he didn't leave permanently until September 22. Like D1, he went from local travel along simple paths to long distance travel that included looping flight. Unfortunately, he died of electrocution on November 6 near Rockford, Iowa. The loss of D14 was tragic for personal and research reasons. Everything we could have learned from him ended in his premature death.

As their flight paths show, D1 and D14 spent some time acquiring skills and strength before dispersing. They needed to be able to fly strongly, work with the wind, find food, and navigate. We have recently begun seeing some increased complexity and distance in Four's flights. Only time will tell if and when she disperses, but Four's flight paths and recent calf-eating excursion seem like good signs. Stay tuned! You can make your own D1, D14, and Four maps here:

Four, 9/1/14 to 9/17/14

On Migration

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 our eagles with tracking devices in part to help answer some of these questions. To date, we've learned D1's migratory paths and summer and winter ranges, observed an interesting difference in dispersal patterns between D1 and D14, and an interesting similarity between D14 and Four. 

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, Snowy Owls, and Bald eagles taught me otherwise.

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 (although 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.

D1's looping migratory path in 2012 and 2013
I also thought that birds used to travel the same path backwards and forwards. However, it turns out that many birds, including our own D1, favor loop migrations. This pattern might be a response to seasonal prevailing winds, food availability, tradition, and/or genetic influence. More to the point, loop migrations require more complex navigational skills (and perhaps inheritances) than a simple point A to point B path.

So how do birds navigate? Migration studies have found five 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.
  • Wind patterns: A recent study at Cornell using crowdsourced data found that passerine birds are heavily dependent on wind direction for migration. By shifting routes, birds take advantage of stronger tailwinds in spring and less severe headwinds in fall.
  • 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. While we haven't yet seen Four in the company of eagles outside her family, she's only begun her wandering. Since young bald eagles are much more nomadic than adult Bald eagles, we hope to have several more years of watching her. 

In addition to watching our eagles, keep an eye on the animals outside your window. We know fall is here, and so do they.
  • Birds may begin gathering in huge premigratory flocks, gorging on food to build fat reserves, and exhibiting restlessness, also known by the marvelous word zugunruheThese behaviors help prepare birds for migration. Keeping a well-stocked bird feeder helps them too.
  • Juvenile peregrine falcons are on the move! You'll sometimes catch a stranger on cam as they disperse down river. Please let us know if you get any band numbers! 
  • Frogs and turtles may begin migrating from summering breeding grounds to deeper bodies of water. While their travels are short, they still fit the definition of migration. For more on frog hibernation, click here. The Minnesota DNR has a very good article on helping migrating turtles cross roads, some cities close roads, and volunteers help keep migrating frogs safe. 
  • Some insects migrate and others hibernate. Aggregating paper wasps may be getting ready to hibernate, while dragonflies migrate in large numbers (or check this source out).
  • Watch out for migrating Mule deer! According to National Geographic, they make a 150-mile migration twice yearly from the low desert to the high mountains.
Have a happy fall and take the time to watch for migrating birds and other animals! Would you like to know if the winds are favorable for bird migration in your area? Check these links 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?