Monday, February 23, 2015

Eggs, eggs, eggs!

Egg Questions and Answers

How long does it take a bald eagle egg to hatch?
There are two ways to think about this: from egg laying to hatch, and from pip to hatch
  • From egg-laying to hatch: Most experts say 35 days, give or take a few. In Decorah, hatches have ranged from 35 to 39 days after lay, with the first egg usually taking the longest. I'd look for first hatch to occur about 38 days after lay, so we're looking at March 28.  In Fort St. Vrain, hatches have ranged from 36 to 40 days after lay, with the first egg usually taking the longest, so we're looking at March 25th. But it could be a little earlier or a slightly later than that.
  • From pip to hatch: Pipping occurs when the baby eagle's egg tooth first breaks through the eggshell. It can take as long as a day for the chick to fully hatch. 
How big is an eagle's egg?
Bald eagles lay white, oval shaped eggs. Size-wise, they are just a tad smaller than a tennis ball, although tennis balls are round, not oval. They weigh approximately 125 grams or 4.4 ounces, and are on average about 2.9 inches long and 2.2 inches wide.

How many eggs will the eagles lay? Will all of their eggs hatch?
Although the most common clutch size for eagles is two eggs, the Decorah Eagles and Fort St. Vrain  have a history of laying three eggs. The breakdown among bald eagles in general is as follows:
  • 79% of clutches have two eggs
  • 17% of clutches have one egg
  • 4% of clutches have three eggs
Both sets of eagles have hatched all of their eggs to date; however, fertilized eggs can fail to develop due to extreme cold, soft shells, or microorganisms. We are hopeful they will have another great year, but we won't know until it happens.

What is in those eagle eggs?
At this point, the developing embryo is still quite small. It is surrounded by the amnion, a fluid-filled bag that helps protect it from jarring and sloshing. The yolk and albumin provide nourishment and additional cushioning, the allantois filters out waste products from the kidneys, and the chorion provides ventilation.  Inner and outer shell membranes help safeguard the embryo from bacterial contamination and keep the egg's insides from leaking out through its highly porous surface.  Embryo, fluids, and membranes are all enclosed in a chalky membrane that strengthens the egg, providing an additional layer of protection from punctures and pressure. An outer cuticle on the chalky membrane (what we think of as the shell) gives texture and even more protection to the eggs.

How do eagle parents care for eggs?
Eagle parents ensure optimal temperature and humidity by alternately incubating, getting off, and rearranging their eggs. While turning eggs might be a matter of instinct, it also prevents the embryos from sticking to the insides of their eggshells, and it optimizes membrane growth. They move very carefully around the eggs, often balling or partially closing their long, sharp talons to keep from puncturing the eggs. Concerned about the amount of time the eagles are spending off the eggs?
Read this blog:

The following resources helped me learn and write about this topic:
  • Bald eagle egg size and color:
  • What's inside that egg?
  • The importance of egg-turning:
  • Previous blogs at this site.
Did you know?
Some birds bury their eggs in compost heaps to incubate them. Meet the megapodes!

Tuesday, February 17, 2015

Sex determination in birds

Humans, other mammals, and some insects genetically determine sex via X and Y chromosomes. In the XY sex-determination system, females inherit an X chromosome from each parent, while males inherit an X-chromosome from their mothers and a Y-chromosome from their fathers. Since homogametic XX females produce only X chromosomes, heterogametic XY males dictate the sex of their offspring.

Human watchers (including myself) often apply sex-based characteristics to our beloved eagles. Mom and Dad appear to have many of the same housekeeping arrangements and arguments that we do, so it might be surprising to learn that they turn our XY sex-determination system on its head!

Introducing...the ZW sex-determination system!
Birds, some reptiles, some amphibians, and some fish replace our X and Y with the ZW system. Let's look at a few differences:
  • I'm a female homogametic XX and my husband is a male heterogametic XY, but female birds are heterogametic ZW and male birds are homogametic ZZ. Among other things, this means that Mom's ova determines the sex of the little E's (or D's) we coo over in Decorah and elsewhere. 
  • XY and ZW chromosomes come from different areas of the genome. That means that our system for genetically determining sex is not very closely related to a ZW'er's system for determining sex, even though they both work roughly the same way. 
  • ZW sex determination is dose-dependent, at least in birds. Z doesn't create a male in and of itself. Two Z's (ZZ) are required to make enough male-determining product. Females may be female not because of the W, but because they lack two Z's.
Having said that, ZW females still produce eggs and ZZ males still produce sperm. There are some other similarities between us and them:
  • The non-recombining Y and W chromosomes have both degenerated over time. Today, the mammalian X carries over three times more genes than the Y does, whereas the chicken Z carries over ten times more than the W. Like our Y-chromosome, the W doesn't combine well with its opposite. Deletion, degradation, and mutations occur more quickly on the Y and W than on the X and Z. 
  • Y and W resemble X and Z more closely in 'primitive' animals and plants than in those that have evolved more recently, and both Y and W continue to shrink. Will human maleness and bird femaleness disappear? I don't think it's likely, but some people believe that new forms of sex determination might arise from somewhere else in the genome.  
How does the ZW system affect birds?
We know birds aren't human, even though we might call them Mom and Dad, think of them as friends, and invest them with human-like feelings, duties, and housekeeping systems. Still, it seems odd that they have a system of genetically-determined sex that is so different than ours. How does the ZW system affect them? 
  • Z chromosomes contain more genetic information than W chromosomes. Since ZZ male birds have two Z chromosomes, they are more likely to pass on sex-linked traits than ZW female birds. For example, Decorah daughter (ZW) Four got a Z from Dad and a W from Mom, while son (ZZ) Decorah got a Z from each parent. A male bird contributes a Z to sons and daughters, while a female passes a Z to sons only. Since Dad's Z contains a lot more information than Mom's W, Dad contributes more genetic information on that particular chromosome, especially when it comes to daughters. In humans, females conserve more sex-linked traits than males, and pass them on to male and female children. The opposite is true in birds. 
  • What about that flashy male plumage? We know that male birds conserve more sex-linked traits and pass them on to sons and daughters, but male plumage is more complicated than it appears.  Recent work published in the journal Evolution indicates that female birds were once as flashy as males. We think that sexual selection drove male color evolution (females prefer colorful males), and natural selection drove female loss of color (brighter females and young were more likely to be spotted by predators and competitors). 
  • So what role did the ZW system play? A couple of ideas. W has degraded over millions of years and carries much less information than it did previously. If the gene for female color was located on W, it may have been lost somewhere in the past. Alternatively, Z is more highly expressed than W. The resulting imbalance in hormonal secretions between lower ZW females and higher ZZ males drives fundamental sex differences in phenotype, development, and physiology. A ZW chromosome may not provide enough oomph to turn bright plumage on elsewhere in the genome, assuming it exists in female birds.
  • What about birds of prey? Unlike most birds, female raptors or birds of prey are larger than male birds of prey. And unlike most birds, male and female raptors sport the same plumage. With the exception of kestrels, we can't use plumage color to tell males and females apart. What happened to de-link plumage color with sex and why is size, which is still linked to sex, inverse almost uniquely among birds of prey? Is it expressed in the Z or W chromosomes (W chromosomes are 'large in many raptors'), or elsewhere in the genome? Curious minds want to know!  
Why do we have XY and ZW?
No one knows for sure. All snakes and all birds use ZW determination, and all mammals use XY determination (even when it gets weird, as it does with platypus and voles, X is always present). But fish, amphibians, turtles, and lizards might use XY, ZW, or temperature-dependent selection (TDS) depending on the species. The switch from a variety of systems (ZW, TDS, XY) appears to have occurred around the time the ancestor of all reptiles split from the ancestor of all mammals, but it's not yet clear whether there was an intermediate stage between ZW and XY systems, or whether the transitions occurred directly.  

Is the ZW system really that different? I don't think so. While lovely heterogametic ZW Mom determines sex and handsome homogametic ZZ Dad passes more genetic material to his offspring, males still produce sperm, females still produce eggs, and genetically determined sexes are still differentiated by phenotype, development, and physiology. Whatever the reason for XY and ZW, us XY'ers at the Raptor Resource Project wish all birds the best of luck this spring!

A list of resources that helped me learn about this topic:
Did you know?

Friday, February 06, 2015

Line-Up: Barred Owls, Great Horned Owls, and Bald Eagles

At A Glance
How do our birds compare? While bald eagles are clearly the heavyweights of the group, barred owls and great horned owls are territorial and can be quite aggressive. All three are generalist feeders. Bald eagles build stick nests, while barred owls and great horned owls nest opportunistically.

Barred Owls
Great Horned Owls

Bald Eagles
16.9–19.7 in
43–50 cm
18.1–24.8 in
46–63 cm
28–37.8 in
71–96 cm
39–43.3 in
99–110 cm
39.8–57.1 in
101–145 cm
80.3 in
204 cm
1.03–2.3 lbs
470–1050 g
2.0–5.5 lbs 
910–2500 g
6.6–13.8 lbs.
3000–6300 g
Small mammals, birds, amphibians, reptiles, fish, and invertebrates. 
Small to medium mammals, birds, reptiles, insects, fish, invertebrates, and sometimes carrion. The Owl Pages states GHO can take prey 2-3 times their size.
Fish, birds (especially waterfowl), small to relatively large mammals, reptiles, amphibians, crustaceans, and carrion. 
Lays eggs January to April (Iowa). Nests in cavities, other birds' nests, squirrels' nests, and nest boxes.
Lays in February (Iowa). Nests in other birds' nests, squirrels' nests, cavities, dead snags, deserted buildings, cliff ledges, and human made platforms.
Lays eggs mid-February to early March (Decorah). Builds large stick nests and nests in them for multiple years. 
Mixed forests of large trees, preferably near water.
All over, although they have a preference for spaces that combine open habitat with forest.
Forests of large, mature trees adjacent to bodies of water 

This line-up makes it pretty clear that the area is as desirable to owls as it is to eagles. Like much of the #driftless, the area encompasses forest, blufflands, running water, ponds, and open spaces. Food is abundant and nests and potential perching spaces abound.

We know how eagles and owls interact. But how do great horned owls and barred owls interact? It looks like great horned owls are a bigger problem for barred owls than they are for bald eagles. The good people at Cornell Lab of Ornithology write that ...A barred owl's most dangerous predator is the Great Horned Owl, which eats eggs, young birds, and occasionally adults. A video made by Dragonlainey on the night of February 3rd features the sounds of great horned and barred owls. It opens with great horned owl calling. Instead of the familar pair hooting, this sounds a bit more like a harnk call - another great horned owl call that isn't especially well-understood (this is a guess on my part and I will add more information if I get it). Familiar GHO hooting starts at about 1:05 into the video, and at about 3:15, we hear an unearthly noise that sounds a lot like the alarm call of barred owls as recorded at The Owl Pages.

Could the great horned owls displace the barred owls or vice versa? The jury is out on that question. Cornell writes that barred owls are...Territorial all year round, Barred Owls chase away intruders while hooting loudly. They are even more aggressive during nesting season (particularly the females), sometimes striking intruders with their feet. We've seen the barred owls attack the eagles in the nest before, and barred owls are displacing spotted owls in the northwestern United States, but I couldn't find any evidence of them displacing great horned owls. While Cornell mentions that great horned owls can shift barred owls on their range, I couldn't find any documentation of outright displacement there, either. I did learn that barred owls are real homebodies. In a study of 158 banded barred owls, researchers sound that none of them displaced more than about six miles away. Compare that to D1's 900+ mile odyssey, or even Four's 100-mile journey!

At present, I suspect the owls and eagles will live uneasily together in inter- and intra-order competition. If we can find and reach the barred owl nest, it would be interesting to band the young and study the family's movements now that great horned owls have arrived.

Additional Owl blogs from RRP
Things that helped me learn about this topic
Did you know?

Tuesday, February 03, 2015

How Do The Eagles Stay Warm?

Mom, Dad, and eaglets in a late storm, 2011
We get a lot of questions about bald eagles and cold weather. I've written a few posts on the subject, but thought I'd combine them here. Unique body features and changes in physiology and behavior help bald eagles maximize energy gain, minimize energy loss, and incubate eggs in cold temperatures.

To maximize gain, eagles forage in groups, gorge food, and increase the assimilation of ingested food energy. To minimize loss, they become sedentary, seek protective microclimates, and reduce night-time body temperature. Put simply, successful eagles use the least amount of energy to get the most amount of food. Here's how they do it.
  • Foraging in groups (or observing and following other birds on their territory) increases the likelihood that eagles will find food with less energy expenditure than if they hunted alone. Bald eagles steal prey from other eagles and birds (kleptoparasitism), an optimal behavior, at least during periods of food scarcity, for animals that forage together. 
  • Gorging food helps eagles load calories quickly and reduces the likelihood of food theft by another animal.
  • Cold weather causes changes in blood flow. Less blood flows to skin and extremities, making more blood available for visceral organs like the stomach.  This helps eagles reduce energy radiation and increase the assimilation of ingested food energy at a time when every calorie is crucial. 
  • It takes energy to fly, procure sticks, and nestorate. Reducing activity reduces energy consumption and slows metabolism.
  • Sheltering in protective microclimates (thick brush, bluff pockets, sheltered branches, coniferous trees) reduces wind exposure. These microclimes also holds heat more efficiently than open spaces, slowing energy radiation and minimizing loss. 
  • Bald eagles reduce their body temperatures at night an average of 1.8 degrees. This slight hypothermic condition reduces the temperature gradient between their body and the environment, letting them burn fewer calories to stay warm. 
Eagles also have physical adaptations that help them stay warm and incubate their eggs. 
  • Feathers are wonderful insulators. The roughly 7,000 feathers an eagle grows help keep it warm and dry. Stiff exterior vane feathers zip together over smaller, fluffier down feathers, providing an overcoat of sorts to shed water and help keep heat from escaping. Underneath, down feathers trap pockets of air next to the eagle's body, where it is quickly warmed and prevented from escaping. 
  • However, some heat needs to escape during incubation. A brood patch is a patch of bare skin on an eagle's breast. Eagles can roll, wiggle, and tuck their eggs up against the brood patch, effectively transferring heat from the eagle's roughly 104-degree body to the eggs. Since the optimal temperature for incubation is 99 degrees, some heat can be lost. The eagles regulate egg temperature by alternately incubating the eggs and leaving them uncovered. 
  • An eagle's large size helps it retain heat. Eagles have a higher inside-to-surface area ratio than a smaller bird like a chickadee. Less surface area means less heat lost to radiation, which is why, within a broadly distributed group of animals, northern animals tend to be larger than their southern cousins. (
  • An eagle's legs use counter-current heat exchange to control body temperature. Warm arterial blood flowing from an eagle's core into its feet passes cool venous blood flowing the other way. Heat is exchanged, warming the blood flowing into its core and cooling the blood flowing into its feet. The cooler blood is still warm enough to prevent frostbite, but the lower temperature reduces the gradient between its insides and its outsides, preventing excessive heat loss through its feet. 
  • An eagle's leg muscles are tucked up under its feathers, nearer the warm center of its body, and it has very few soft tissues in its long legs and feet, which are wrapped by thick, scaly skin that helps protect them from the cold. If its feet do get cold, it can always tuck them, often one at a time, underneath its feathers. 
It's hard not to be concerned about eagles and other wildlife during extreme cold events. But eagles and other animals that live outdoors are well-prepared to deal with them. You can help by keeping seed and suet feeders stocked, keeping water available, and providing shelter for birds. The Minnesota DNR offers these winter feeding tips. Our pinterest has some cute ideas, with more to come: 

More information on Eagles and Cold

Ecological Energetics and Foraging Behavior of Overwintering Bald Eagles
Mark V. Stalmaster and James A. Gessaman
Ecological Monographs
Vol. 54, No. 4 (Dec., 1984), pp. 407-428
Published by: Ecological Society of America

Food Consumption and Energy Requirements of Captive Bald Eagles
Mark V. Stalmaster and James A. Gessaman
The Journal of Wildlife Management , Vol. 46, No. 3 (Jul., 1982) , pp. 646-654
Published by: Wiley on behalf of the Wildlife Society

A Great Read

Winter World: The Ingenuity of Animal Survival by Bernd Heinrich

Did you Know?

Temperature Rhythms Keep (Human) Body Clocks in Sync:

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!