Birds on Radar, October 3rd, 2017

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

After three nights and days of rain, storms, and winds from the south, last night was finally clear and calm. The full moon was so bright that no light was required as I walked up and down my driveway and around our yard. While I couldn't see any birds, a few short flight calls overhead told me that a great river of birds was flowing in the sky above me.

Bald eagles and other bird of prey migrate during the day, when they can take advantage of thermal soaring to help lift and carry their large bodies long distances. But many songbirds migrate in large flocks or swarms at night. Avian predators are less active, skies are often less turbulent, the cooler night air is slightly denser and helps birds dump heat and like sailors, birds can use the stars for navigation across an otherwise relatively featureless landscape.

How many birds were migrating last night? Two words: Green Doughnuts.



The map above shows reflectivity, defined as the amount of transmitted power returned to the radar receiver after hitting precipitation, compared to a reference power density at a distance of 1 meter from the radar antenna. In short, when radar hits stuff, it gets bounced back. The more radar that gets bounced back relative to the reference, the more intense or denser the stuff it is hitting: precipitation, flocks of birds, migrating bats, insects rising into the sky, and so on.

If you look at the map above, you can see a line of storms moving to the east. Some of these storms are dropping quite a bit of precipitation, as shown by the yellow and red colors in Michigan's Upper Peninsula. But you can also see blue and green doughnuts or blooms blowing up around sunset (which occurred at 6:47pm CT in eastern Minnesota and Iowa last night). The unique pattern, lower reflectivity, behavior (an ebb and flow that begins at sunset and begins to peter out at about 1:30am), weather, and time of year all tell us that these are migratory birds.

So why was I excited about green doughnuts? Check out the reflectivity key below...

The colors represent the strength of returned energy to the radar expressed in values of decibels (dBZ). As we stated earlier, the amount of radar reflected back to the ground unit is proportional to the number and diameter of stuff - drops/flakes/birds/insects - per unit volume. The US Weather service defines rainfall rates and types here, while the Cornell Lab of Ornithology defines bird 'rates' (but not types) as follows:

• Minimal migration: < 5 dBZ — fewer than 59 birds per cubic kilometer
• Light migration: 5-10 dBZ — approximating 59-71 birds per cubic kilometer
• Moderate migration: 10-20 dBZ — approximating 71-227 birds per cubic kilometer
• Heavy migration: 20-30 dBZ — approximating 227-1788 birds per cubic kilometer
• Extreme: >30 dBZ — more than 1788 birds per cubic kilometer (actually  occurs at some times in very rare circumstances)

Blue doughnuts are pretty typical, at least during the summer months. But green doughnuts are something else! Last night's radar shows heavy migration that approached extreme migration in some places. As I walked and listened, I thought it was probably a pretty special night. I can't believe how special it really was! I'll be out looking and listening for birds again tonight.

Additional Resources
How do normally diurnal birds become temporarily nocturnal? Read more about that here (warning - this is a dense read): https://www.nature.com/articles/srep34207

The Cornell Lab of Ornithology publishes bird forecasts and migration reports on their website at http://birdcast.info/. Check it out and keep those feeders filled for hungry seed, nut, and suet eaters that are resting and refueling in your yard!

BirdNote did an excellent story about nocturnal migration. You can listen to that here: https://www.birdnote.org/show/nocturnal-migration-songbirds.

The Old Bird studies nocturnal flight call activity. While he isn't the only one, almost everyone involved in this activity cited him as the original. Check out his website at: http://oldbird.org/.

Thanks to the University of Wisconsin, we have a national radar map here: http://www.raptorresource.org/maps/UW_Composite_Radar/ and one showing migration for the great lakes/upper midwest here: http://www.raptorresource.org/maps/UW_Composite_Radar/. These maps archive data over several months. We encourage you to check them out and compare nights - summer versus winter, stormy nights versus calm nights, and good flight nights.

Good luck birds, and we'll see you next spring!

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. 

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/

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. http://www.nytimes.com/2012/04/27/science/study-sheds-light-on-how-pigeons-navigate-by-magnetic-field.html?_r=0
• 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. http://www.birds.cornell.edu/AllAboutBirds/studying/migration/navigation
• 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 zugunruhe. These 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: 

• Wind Map: http://hint.fm/wind/
• Intellicast Wind Map: http://www.intellicast.com/global/wind/current.aspx?region=namer

---

Some things that helped me write this post: 

• Migrating raptors of the world: their ecology and conservation. Keith L. Bildstein
• Living on the Wind: Across the Hemisphere with Migratory Birds. Scott Weidensaul
• All About Birds: Navigation -http://www.birds.cornell.edu/AllAboutBirds/studying/migration/navigation
• Weather patterns aid small birds: 
  http://www.news.cornell.edu/stories/2014/05/weather-patterns-aid-small-birds-migrations
• Lissajous patterns. It struck me that the eagles' loop travels somewhat resembled a Lissajous pattern. A little googling found this very cool project: http://kaurov.com/wordpress/?p=692

* 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? 

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 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. http://www.nytimes.com/2012/04/27/science/study-sheds-light-on-how-pigeons-navigate-by-magnetic-field.html?_r=0
• 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. http://www.birds.cornell.edu/AllAboutBirds/studying/migration/navigation
• 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:

• Migrating raptors of the world: their ecology and conservation. Keith L. Bildstein
• Living on the Wind: Across the Hemisphere with Migratory Birds. Scott Weidensaul
• All About Birds: Navigation - http://www.birds.cornell.edu/AllAboutBirds/studying/migration/navigation
• Migration: http://www.nps.gov/akso/parkwise/students/referencelibrary/general/migrationbasics.htm

* 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?