Report on the Unhatched Eagle Egg from N2B

Unhatched egg, N2B

We received the results on the unhatched egg from N2B. As watchers might recall, one of the eggs failed to hatch. We thought the embryo might have died early in formation, but according to tests run by Iowa State University, the egg was never fertile. How could that be?

A quick primer on egg fertilization in birds. Sperm needs to encounter an ovum at the infundibulum, or site of fertilization.  If sperm are too early, they will die prior to the arrival of an ovum. If sperm are too late, they can't penetrate the eggshell layers that form around the ovum in the female's oviduct. So how do birds assure fertilized eggs? They:

• Copulate regularly. Regular copulation helps assure a good supply of sperm - especially important in an animal that regularly clears its cloaca when eliminating waste!
• Store sperm. Sperm storage tubules maintain sperm viability, prevent stored sperm from being ejected, and continuously release sperm to the infundibulum.
• Concentrate sperm at the infundibulum. Released sperm are passively carried to the infundibulum. Their continuous release and relatively slow drift help ensure that sperm are present when an ovum arrives. 

After removal from N2B

It is good to know that egg number one didn't contain an embryo, but why wasn't it fertilized? Three suggestions based on a simple idea: sperm wasn't present at the infundibulum when the first ovum arrived. 

• While eagles have been observed copulating ten months out of the year, males don't produce sperm year round and they don't store it very long once production is underway. Perhaps Dad wasn't producing sperm in time for Mom's first egg. Sperm production is not required for pair bonding.
• Mom may not have had enough sperm stored to concentrate sperm at the infundibulum in time for ovum #1.  
• Age might be impacting reproductive success in either Mom or Dad. While free-living animals don't tend to have 'menopause' - a long stretch of time in which they do not bear young - age does impact fertility.

Some watchers have expressed concern that age might be a factor in the failure of egg number one. It isn't especially likely in Mom based on what we know. In general, a female bird that produces a healthy, intact egg is most likely fertile. Reduced fertility in our 'elderly' female peregrine falcons tends to be accompanied by changes in the amount of eggs laid, egg color, shape, and condition. Eggs might be unusually colored, pitted, or shaped - all things we saw at Xcel Energy's Sherco facility in 2014 and 2015. While we don't have a lot of data about senescence and egg production in bald eagles, wild eagles are generally assumed to live for 20-30 years. Given that Mom is just fourteen years old, age-related fertility impairment seems unlikely. 

So how about Dad? We believe he is at least 19 years old, although we don't know exactly how old he is. Senescence and sperm production in birds is a little complicated. Research indicates that aging impacts sperm quantity, quality, and motility in birds, and eggs fertilized by older males hatch at reduced rates when compared to those fertilized by younger males. But even with reduced motility, the sperm of older males tends to perform better than the sperm of younger males in a female bird's body. In short, older birds have less sperm than younger birds, and the sperm they have is less motile and of lower quality. But studies have found that older birds are more likely to fertilize eggs than their younger counterparts, even if those eggs are less likely to hatch.

Why are older male birds more successful at fertilizing eggs? The study Senescent sperm performance in old male birds found that obstacles to sperm movement in a female bird's reproductive tract affected older males less than younger males. I would love to see research on the role that skill and pair bonds play in sperm retention (I am defining 'skill' as actions taken by the male to assure a high degree of receptivity in his partner). We know that at least some female birds are able to preferentially reject the sperm of less desirable males, that female birds who mate with familiar males often produce more fertilized eggs with more egg mass than those who mate with novel males, and that the success of novel mating is highly dependent on male behavior. Reproduction is clearly much more complicated than we used to think, and it is obviously past time to drop the pejorative term bird-brained!

Having said that, we are back to the question of Dad's fertility. One infertile egg doesn't really give us enough data to come to any conclusions, but it is very helpful to know why the egg didn't hatch. We will be documenting whether or not nest production continues to decline at N2B. Thanks to John Howe, Kike Arnal, Pat Schlarbaum, Dr. Ensley, and Iowa State University for giving us more insight into the lives of the birds we follow. We are hoping for the best for our beloved Mom and Dad!

---

Did you know?

• Sperm competition can result in speedier sperm among animals that have multiple mates while ovulating. Female mice, for example, take multiple mates and can't reject or store sperm long, so speed is important! http://www.sciencemag.org/news/2014/04/scienceshot-how-make-speedy-sperm
• Why didn't the egg explode? A decomposing embryo rapidly produces gasses that can explode an egg, but this egg didn't contain anything except (presumably) Mom's blastodisc. It was also kept relatively cool and protected from direct sunlight by grass and nest detritus. John really had to dig for it! He told us that he dug through roughly a 5-gallon peregrine gravel pail worth of stuff to find the egg, including a squirrel's skull!
• Would a receptive female in a long-term pair be likely to produce more eggs? I had very little time to follow up on the question, but here is one study that touches on it:
  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817173/

Things that helped me learn about this topic

• Senescent sperm performance in old male birds:
  http://onlinelibrary.wiley.com/doi/10.1111/j.1420-9101.2008.01650.x/full
• The sperm of aging male bustards retards their offspring’s development (I'm linking straight to the discussion, but the whole paper is available):
  http://www.nature.com/articles/ncomms7146#discussion
• How to make speedy sperm!
  http://www.sciencemag.org/news/2014/04/scienceshot-how-make-speedy-sperm
• The risk and intensity of sperm ejection in female birds (this needs to be purchased for full access): http://www.journals.uchicago.edu/doi/full/10.1086/661244
• Learning enhances female control over reproductive investment in the Japanese quail (full article): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2817173/

So does this mean it is clearly the magic in Dad's many, many sticks? ;) All jokes aside, Dad spends a great deal of time working on the nest and providing food, although Mom is more than capable of building and hunting on her own. We tend to define the reproductive season as starting at the beginning of active copulation or egg-laying. But today's daily activities help cement the bond between Mom and Dad and may result in increased productivity months from now: more fertilized eggs with higher egg mass, more hatched eggs, and heavier weights in nestling eaglets during critical stages of growth. We are learning that eagle reproductive success depends on far more than the brief period of time they spend engaging in productive copulation.

Birds and Nest-Building

This blog was inspired by the paper 'The design and function of birds' nests', Ecology and Evolution 2014; 4(20): 3909–3928. It can be found here: http://bit.ly/1Q55LZd

Hard at work on N2B

What determines the design of a bird's nest? You know the type of nest I'm talking about, right? A bald eagle's large platform nest, built with sticks, high up in the branches of a tree.  Or maybe a scrape created in dirt or gravel on a shallow cliff ledge by nesting peregrine falcons. How about the burrows created by bank swallows and belted kingfishers, or the cavity nests excavated in trees by woodpeckers? Or am I thinking of the pendulant and sometimes very elaborate suspended nests created by orioles and weavers?

When I think of bird nests, I think of the classic small cupped nests I collected as a child. But birds nest in a variety of ways and places, creating nests from twigs, sticks, moss, grass, fur or hair, roots, bark, leaves, pine needles, feathers, lichens, paper, insect cocoons, snakeskin, saliva, and other materials. They nest high in trees, but they also nest on the ground, under the ground, in the darkness of chimneys and caves, and even in active wasp nests. Some birds construct elaborate nests while others get by with a shallow scrape in a stolen or abandoned nest.

Nests traditionally have been thought of as nothing more than places for birds to lay eggs and raise young. It was assumed that natural selection and the need to minimize the risk of predation determined the design of completed nests. However, recent research has found that sexual selection, parasites, and the need to build a nest that can handle variation in weather conditions (which we certainly see in Iowa, Colorado, and Minnesota!) also play significant roles in influencing nest design.

Sexual Selection
How does sexual selection play a role in nest design? It goes something like this: birds that build awesome nests relative to other members of their species must be healthy, high-quality mates. Think about Decorah. Mom and Dad spend a lot of time procuring and hauling wood and soft materials into the nest. This is a costly activity energetically speaking, and it requires both eagles to be in good shape - well-fed (which implies provisioning ability), skilled at flying, and relatively parasite-free. While different things might indicate quality to different species of birds, it appears that large nests signify quality among bald eagles.

Bald eagle nests are bi-parentally built - that is, both parents work on them. However, in Decorah it is Dad who spends the most time getting the sticks just right, scraping in the nest bowl, and preparing the egg cup. Studies have found that female magpies adjust their reproductive effort in relation to the male's nest-building activities. The more time he spends working on the nest, the more time and energy she puts toward reproductive processes, including copulation.

What could we watch for in our eagles? A study of male house sparrows found that they call to females when adding feathers to the nest, suggesting that they wish the behavior to be observed. The volume of feathers delivered by males was positively correlated with clutch size and female provisioning rates. While Dad doesn't usually vocalize when he places a stick, he often interacts with Mom in some way - taking a stick from her, moving sticks while she is in the nest, and so on. His obsession with proper placement might be his way of signaling fitness and getting more tailfeather from Mom to boot! I'm going to pay more attention to Mom and Dad's interactions during and following their sticky adventures!

While the paper doesn't touch on intelligence, it is hard for me to believe that it isn't part of the total sexual selection package. Building a nest is a lot harder than it looks and requires a whole host of things: access to food (which implies hunting ability and understanding of a territory, among other things), strength (which implies access to food and a body relatively free of parasites), and flight skills (ditto). An intelligent bird will presumably make better decisions about maximizing energy gain, procuring food, and hauling and placing sticks. Its intelligence should help it live longer and build a sexier nest, which in theory will lead to more copulation, more fertilized eggs, and offspring that inherit Mom and Dad's traits.

Parasites
Bird parasites include lice, fleas, flies, mites, ticks, leeches, fungi, and bacteria. We've seen firsthand the damage that blackflies and hippoboscids can do: killing young falcons, driving young from the nest to early, and delaying development. Birds have a wide variety of responses to parasites, including molting feathers, the use of feather toxins, preening, and maintaining their nests.

Watchers might remember Stitch and Spot, the red-tailed hawks from Eaglecrest Wildlife. Male Spot brought green plant material into the nest on a regular basis. The greenery (probably blue oak branches) contained high levels of monoterpenes and isoprene, which helped reduce parasites and fungi and may also have signaled fitness to female Stitch in the same way that a large nest might signal quality to a female eagle [a blog on greenery and parasites can be found here].

However, the bald eagles we watch don't appear to intentionally carry greenery into their nests. So what do they do? Many watchers have commented on the number of mice in the nest. For the mice, an eagle's nest offers food (scraps of protein and stomach and crop contents of prey), shelter, and safety from serious mouse predators like fox and owls. While bald eagles would eat a mouse, it isn't much more than an hors d'oeuvres compared to prey like pigeons, trout, squirrels, and so on, so the risk is well worth the payout! In return, mice devour leftovers and help keep the nest a little cleaner and presumably parasite-free than it would be otherwise. As with greenery and hawks, sexual selection might drive a nest-building behavior that also repels undesired guests!

An early nesting season also helps reduce parasitization of young, although it means parent birds need to be ready for the challenges that winter brings. This leads us to...

Environmental Adjustment
Remember 2013? The birds we watch produced young in the teeth of the worst winter since the 1970s. Our eagles were successful in part because the design of their nest influences the microclimate in which eggs and young are incubated. Dad's attention to the nest bowl - rolling, scraping, nest digging, and the careful arrangement of soft materials to form a cup once the substrate is suitable - may both excite Mom and result in a structure that can weather rain, snow, and serious sub-zero temperatures. Of course, if you watch the eagles, you have probably already realized that the types and placement of materials aren't accidental. We've seen the nest bowl and cup carefully prepared ahead of time, and we've seen Dad bring grasses and husks to tuck around Mom while she incubates eggs.

So the next time you watch a bird build a nest, either online or off, remember that much more is going on than meets the eye. Sexual attraction, courtship and flirtation, proof of fitness, and protection of young from parasites and weather all appear to play important roles in the design of finished nests.

---

Things that helped me learn and write about this topic:

• Paper: The design and function of birds' nests', Ecology and Evolution 2014; 4(20): 3909–3928. It can be found here: http://bit.ly/1Q55LZd
• The classification of bird's nests. There are a number of resources, but I thought this one was fun: http://www.backyardnature.net/birdnest.htm
• The 16 most amazing bird nests: http://www.bbc.com/earth/story/20150307-the-16-most-amazing-bird-nests

Did you know that?

There is a bird that nests inside a wasp's nest! Follow this link to read more about the Violaceous Trogon and its extreme housing practices!

Update from Decorah

Answers to your four most commonly asked questions, plus a bonus photo at the bottom of the page!

When will Mom and Dad start working on a new nest?
Did someone mention Eagle Time? In general, Mom and Dad start working on their nest sometime in October. Robin spotted Mom near N2B at one point, but I doubt we'll know whether or not they will adopt it for a month or so. For a time-lapse of the nest, check out this video, pieced together from October 2014 through January 2015: https://youtu.be/Yk19jSED4VE. While nest work started in late October, it didn't really kick into gear until maybe mid-November.

Note added on October 10: Mom and Dad have started working on the nest. Visit our youtube channel for more: https://www.youtube.com/channel/UCoC_-rIBtLCzAl5l-834jZQ

What triggers Mom and Dad to start working on the nest? 
I've got a blog on this subject here: http://goo.gl/658U1y. The short version? Two ideas: Daylight length is quite similar in mid-February and mid-October, which 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). 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.

Where are the Tree Amigos?
Given that no one has seen them, D21, D22, and D23 have most likely dispersed. D1 dispersed on August 13, 2011. D14 left twice, once on September 5th and for good on September 22, 2012. Four was a bit of an outlier. She took two longer exploratory flights in October 2014 before leaving for good on October 25. You can look at the flight maps of all three here: http://goo.gl/RfYLLM or explore Four's last month in Decorah here: http://goo.gl/e50WwF

What's this about HD cameras?
It's true - we are moving to high definition! Cameras will be installed or replaced at N2B and N1 later this fall. Watch for announcements in early October!

Four at the Carlson Pond on 9/9/2014

Who is the eagle in this photo?
On this date in 2014, Bob took this photo of Four at the Carlson pond about a mile from the hatchery. This was one of her first 'long' flights and raised our hopes that she might disperse - which she eventually did, although she waited until late October to do so. Look for more 'on this date' photos as we wait to see what the eagles decide to do!

N2B, or the New Decorah Nest

Members and friends of the Raptor Resource Project spent the last two days building an eagle's nest - or what we hope will become an eagle's nest - near the former site of N2. We put together this blog to answer questions about our work and what it means.

What happened to N2?
The tree that N2 sat in was destroyed during a storm at around 3am on the morning of Saturday, July 18. The storm has been reported as either an F1 tornado, straight-line winds, or a microburst. We don't which of the three it was, but it certainly did a lot of damage! 

Were the eagles harmed?
No. The entire family was accounted for not long afterwards.

Why did you build a nest? Don't Mom and Dad usually make their own nest?

A red-tailed hawk nest built by Neil and Laura.
Great horned owl Robbie gave it his seal of approval. Note that
it was built on the ground and lifted into place.

Eagles usually make their own nests, but when Neil suggested building a nest, Bob was immediately intrigued. He greenlighted the project and we began planning it. Could we build a nest the eagles would adopt? Literature and Neil and Laura's experience building red-tailed hawk and great horned owl nests indicated that we could. In addition to being thrilling in its own right, the project has management implications far beyond Decorah. Like our kestrel and falcon nest boxes, it could be a useful tool for increasing, maintaining, or shifting bird populations.

Why are you calling it N2B?
N2B stands for N2Bob. Bob passed away before we had a chance to begin, but we decided to carry forward in his honor.

How did you build N2B?
Neil and Laura have constructed nests for red-tailed hawks and great horned owls, but building an eagle's nest came with a whole new set of challenges. The nest had to be assembled in place instead of on the ground, which required a climbing team. Materials and sticks had to be gathered and hauled up, which required a ground team. The nest needed to be structurally sound, which required careful work and planning, and it had to go up relatively quickly, which required a lot of prep ahead of time.

We made two scouting trips to the site. On the first trip, Dave Kester suggested a cottonwood that had all the features we were looking for: an open flyway, wonderful perching spots, and a good fork in which to start the nest. Neil climbed up into the tree on our second visit to confirm it while the ground team set to work collecting sticks and getting the area ready for action.

Nest construction began on Wednesday, August 19. Neil and Kike (Kee-Kay) Arnal climbed up to the nest and bolted four 2x4s to build a small frame in the fork of the tree. Meanwhile, the ground team collected bundles of sticks from N2 and bags of leafy debris from the forest floor. Sticks were hauled up so Neil and Kike could insert them between the 'rings' of the 2x4s, creating a stable platform for nest building to begin. Once the base was in place, Neil wove more sticks in a circular pattern, building the nest higher. The two periodically dumped leafy debris into the nest to fill and raise it, alternating sticks and leaves in layers to build a strong, stable platform...much like eagles would do.

Working on the frame, photo courtesy Robin Brumm

Adding leafy debris, photo courtesy Robin Brumm

Adding sticks, photo courtesy Robin Brumm

After several hours of work, the nest looked like this (see this excellent Facebook post by Laura for more images)...

We decided it was time to knock off work until the next day (August 20th), when we returned to finish the job. Amy got to go up with Kike to provide some finishing touches. She wove branches around the outside rails and helped add more debris from the forest floor. While individual sticks and leaves might not seem very strong, the outside edges of the nest were strong and stable enough to stand on.

I think we could have stayed in the tree for the rest of the day, but N2B was intended to be a starter or catalyst nest, not a finished product. We rappelled to the ground, leaving nothing behind but a pair of road-killed squirrels per Bob's instructions.

Why did you put the nest up so fast? Will Mom and Dad do any building if they use it?
Mom and Dad started building N2 in October of 2012, which is just around the corner. We wanted the catalyst nest up with time to spare. Our work notwithstanding, the eagles will bring in more materials to N2B should they choose to use it. Nest building is an important part of bonding and our project won't change that.

Did you put cameras up?
We haven't yet. We'll make a decision on cameras in September. In the meantime, we'll be watching the nest. Mom soared overhead to check out our work on Thursday. We hope she approves!

Some links on contructing or erecting artificial bald eagle nests:

• Constructing bald eagle nests with natural materials: http://www.fs.fed.us/rm/pubs_rm/rm_rn535.pdf
• https://sora.unm.edu/sites/default/files/journals/jrr/v026n04/p00266-p00266.pdf
• Bald eagle nest on an artificial tree: https://sora.unm.edu/sites/default/files/journals/jrr/v022n02/p00066-p00067.pdf

A couple of news stories:

• Decorah News: http://www.decorahnews.com/news-stories/2015/08/11587.html
• KTTC Rochester, MN: http://goo.gl/FxScPe

This project would not have happened without the help and support of an awful lot of people. A million thanks to the landowners for giving us permission to undertake this project. A million more thanks to Neil Rettig and Laura Johnson, Kike Arnal, Dave Kester, John Howe, Brett Mandernack, Jim Robison, Randy Christman, Ryan Schmidt, Jason Thiele, Pat Schlarbaum (Iowa DNR), Maggie Jones, David Linton, Rob Horowich, Nora and Joe Hensley, Robin Brumm, Willard Holthaus, and Chris Gourley. If I forgot your name, email me: ries93@gmail.com and I will add it.

Some bonus photos from Laura and Robin...

Ryan, Brett, and Jim cutting boards for the framework (courtesy Laura Johnson)

Dave Kester wearing Bob's photo (courtesy Laura Johnson)

Kike takes a photo, thrills watchers (courtesy Robin Brumm)

More sticks being collected (courtesy Robin Brumm)