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 mammals, birds, amphibians, reptiles, fish, and invertebrates.
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 Bald Eagles Stay Warm In Cold Weather?

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: