The Broken Egg at the Decorah North Nest

We asked our panel of eagle experts about the broken egg at the Decorah North nest. Could egg viability be determined from the video? Did they have any ideas about egg breakage? Was Mrs. North likely to produce more eggs?

In general, the panel believed that the egg’s viability couldn’t be determined from the video, although two of them commented that a darker patch in the draining egg fluid could have been an embryo. At 19 days old, an embryo would still have been quite small and not especially easy to see given the circumstances. Since we couldn’t come to a definite conclusion, our answer is “No” – we don’t know whether the egg was viable or not. We have some information about embryonic developmental stages here: https://raptorresource.blogspot.com/2017/03/whats-inside-those-bald-eagle-eggs.html

So why did the egg break? Female eagles lay down calcium in their shell gland/uterus, and their large eggs take a lot of calcium. Was she not able to lay down enough calcium to protect her egg from breaking, and if so, why? She may not have had enough calcium available during that stage of egg formation, or something else could be going on with her reproductive system. One member of the panel asked about her age. Female bald eagles and peregrine falcons can experience reduced fertility as they age. They might lay fewer eggs, strangely colored eggs, oddly shaped eggs, ‘smooshy’ eggs, thinly-shelled eggs, or infertile eggs that never hatch. It’s too early to know whether that’s the case, but we will be watching to see if any of those things happen moving forward.

Was she poisoned again? We saw no sign that she was poisoned this year, and the poison from two years ago would not affect her shell deposition now.

Is Mrs. North likely to produce more eggs? The answers were mixed. If she does, it should happen around March 30, or about 14 days after she lost the first one. Other birds of prey, including gyrfalcons and barn owls, take 14 days to recycle after losing a clutch. I really went back and forth about this (19 days is a long time to incubate an egg!) but the answer gave me some hope, especially given that Mr. and Mrs. North are still copulating. We’re crossing our talons, hoping for the best, and keeping a very sharp eye on Mrs. North!

DN2 Autopsy Report

We received a final autopsy report on DN2. The eaglet died as a result of Methomyl toxicity, although caffeine was also present. An analysis of DN2’s liver ruled out lead and mercury as the cause of death. It did not include information on the sex of the eaglet.

What is Methomyl?
Methomyl is a broad spectrum insecticide introduced in 1966 to kill a variety of insect pests. It is registered for commercial/professional use on sites including field, vegetable, and orchard crops; sod farms; livestock quarters; commercial premises; and refuse containers. It is toxic enough that products containing more than 1% Methomyl are not available for homeowner or non-professional application.

How were DN2 and Mom North poisoned?
While we don’t know for sure, Methomyl is sometimes used off-label to kill wildlife species that become problematic. 1% Methomyl products including Golden Malrin, Lurectron Scatterbait, and Stimukil are placed in sweet solutions, including caffeinated colas, to poison animals like raccoon that some consider pests and can cause damage to buildings and farm animals. Given the presence of both Methomyl and caffeine, it seems likely that Mom North found a Methomyl-poisoned dying or dead mammal and brought it to the nest to eat. It wouldn’t have taken much to kill DN2 or Mom North, since Methomyl is highly toxic.  Based on what we observed, Mom North was able to metabolize and recover from what she ate, but DN2 was not. While it is possible that the poisoning was the unintended consequence of a legal use of Methomyl, the caffeine suggests it was not.

For more about the problems of baiting wildlife with Methomyl, follow this link: http://www.sciencemag.org/news/2016/02/regulators-move-limit-wildlife-deaths-misuse-deadly-fly-killer

Is this legal?
It is not. While Methomyl and many other pesticides and insecticides are legal when used in a manner consistent with labeling, it is illegal to go off-label and use them in a manner for which they are not intended. Fly bait is legal when used according to direction and illegal used in any other way. There may be state and federal penalties for people caught misusing it, depending on state and federal law, and what species are killed.

What are you going to do about it?
We have already reported the incident to the Iowa State DNR and the US Fish and Wildlife Service, and will report it to the EPA and the manufacturer of the most likely culprit, Golden Malrin. We will also use our RRP website, Facebook and Twitter audience to help spread the message about misuse of Methomyl and let people know that they can petition their states to reclassify Methomyl fly baits as restricted products should they choose to do so. We would love to say that this was an isolated incident, but a quick search for “golden malrin raccoon” reveals just how widespread the practice is. Google it and join the discussion!

Do you know who did it?
No. There is a lot of variability in eagle home ranges. While immediate nest defense/core territories tend to occur within 100 square meters or so of the nest, hunting territories vary considerably.  One study documented bald eagle summer home ranges that varied from 3 square miles to 18 square miles depending on age, nesting status, and available resources. But other studies have found even wider ranges in nesting bald eagles, including 37 square miles for a pair in Louisiana. The landowners in the immediate vicinity of the nest have expressed their commitment to conservation.  With bald eagles searching and scavenging for food over a large area, there is no way to know exactly where it cam from. We are not an enforcement agency, but we have reported what we know to enforcement agencies.

We don’t believe that anyone meant DN2 or Mom North any harm. But people need to know that off-label poisoning kills unintended targets and can have serious legal consequences. We don’t believe that anyone meant DN2 or Mom North any harm. But people need to know that off-label poisoning kills unintended targets and can have serious legal consequences. This may also be a good day to check garages, basements, and storage sheds to see what noxious and toxic herbicides, rodenticides, and insecticides are on hand that have tragic consequences for wildlife, pets, and humans. Want something to replace those highly toxic chemicals? Check out the Bio-Integral Resource Center (http://www.birc.org/) and their list of least-toxic pest control products: http://www.birc.org/2012Directory.pdf. Even if you can't go completely toxin-free, you can reduce harmful consequences by using products correctly and choosing the least toxic option. You can help by sharing and spreading the word to prevent the unintentional killings of bald eagles and other animals.

Note that Methomyl is toxic enough that the Environmental Protection Agency (EPA) and Methomyl manufacturers have agreed to limit use on certain crops to reduce risks to drinking water: http://bit.ly/29yhNcY. EPA and manufacturers have also reached an agreement to stop making and selling some fly bait products and to add information to the label that clarifies the approved uses. EPA believes that these changes will reduce the illegal use of methomyl fly bait products which can kill wildlife, an issue that was reported to EPA by a number of states.

Are you an academic studying this issue? Please contact us by emailing amy@raptorresource.org.

Decorah North News: DN3 has died

Eaglet DN3 died sometime yesterday. Why did DN3 die? While it didn't get nearly as much food as its larger siblings, it was still shooting poop yesterday, indicating that it had enough in its system to be pooping and excreting urates normally. However, DN3 may not have eaten yesterday and didn't eat much the day before that. It also didn't help that the weather was cold and damp. Mom and Dad North are brooding based on the older nestlings, who no longer require daylight brooding unless it is raining, snowing, or unusually cold. DN1 and DN2 have thermal down, but DN3's thermal down was just beginning to come in and it would have benefited from brooding.

We suspect there is no one cause for DN3's death. The six-day difference between DN3 and its oldest sibling resulted in insurmountable developmental differences, including dominance interactions that DN3 always lost given its much smaller size, a resulting lack of food, and a lack of brooding before it was really able to thermoregulate on its own. Had the weather been warmer, DN3 might have been able to survive even given the shortage of food and dominance interactions with the older two, but all three together were too much.

Are Mom and Dad North bad parents? No. Eagles don't fit neatly into our human ideas about what good parents are. The older two have been thriving under their care. From an eagle point of view, there is no purpose in caring for weaker young that are less likely to survive than healthy young. Personally, I would love to have seen DN3 survive and I was very hopeful up until yesterday afternoon. But as someone who loves and watches birds, I also need to accept that this is what they do. Humans don't make good parents for birds.

Why didn't we intervene? What happened at the North Nest is completely normal from an eagle perspective. It is well-documented that the youngest member of a three-nestling clutch can die, even though we haven't seen it in Decorah or Fort St. Vrain (eaglets have died because of hypothermia and illness at FSV, but those were very straightforward deaths and didn't involve just the youngest sibling). We haven't intervened in these situations in the past and will not moving forward.

Watching wild creatures doesn't give us ownership over them. The lives of 'our eagles' are truly their own and this was yet another example of how we differ from them. Juliet Lamb has some excellent perspective on the relationship between watchers and watched here: http://daily.jstor.org/wildlife-cams/


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What is going on at the Decorah North Nest?

What is going on at Decorah North? The oldest eaglet, DN1, is displaying far more aggression than we are used to towards its siblings. It is upsetting to human watchers, especially since little DN3 takes much of DN1's biting, yanking, and twisting. However, we have also seen DN1 and even DN3 attack DN2. In most cases, the aggressive behavior ceases once the target submits, but we have seen biting and bonking continue past submission at least twice.

Why is this happening?
We don't know. Food certainly seems to be part of the puzzle, since we've often seen the eaglets act aggressively towards one another as they establish a feeding-related pecking order. However, we've also seen aggression break out in circumstances seemingly unrelated to food. For example, DN3 attacked DN2 at around 2pm this afternoon, giving it multiple bites, yanks and twists before DN2 submitted. The eaglets had all been fed well earlier today, so why the aggression? Perhaps differences in age and size also contribute to aggression. I would tend to believe that aggressive behavior is hard to stop once it starts, and I'm very curious whether we will see the drop-off in aggression here that we have seen in Decorah.

While siblicide is rare, aggressive behavior is common. It ranges from the relatively mild aggression seen in Decorah and at Fort St. Vrain to the biting, twisting, and yanking seen at Decorah North, Southwest Florida, and several other nests. I think this looks odd and frightening at least in part because Decorah has tended to lack extreme aggression. But Decorah and Fort St. Vrain could very well be outliers when it comes to eagle behavior. At this point, only the eagles know for sure.

Is DN3 growing normally?
It is very tough to tell with nothing but the two 'Cropzillas' to compare it to. DN3 is growing in thermal down to replace natal down, has gotten larger relative to the other two (seemingly overnight!), and is growing footpads. What we've been able to see of its beak has indicated a fairly regular size and shape in accordance with eaglet growth curves, and its talons are appropriately changing to black.

DN3 is just moving into the steepest part of its growth curve. The next five to seven days will give us a look at whether or not it is growing normally. We'll also look for developmental changes like nest wandering, attempting to stand, and tracking Mom and Dad outside the nest. DN3 has already been observed doing some of those things on warm days and we should see them more often as its thermal down grows in.

Is DN2 growing normally?
Yes, DN2 is growing and behaving normally.

Will DN3 survive?
DN3 is strong, gets fed, participates in nest aggression, and submits appropriately. The eaglets tend to fight with their beaks instead of their talons and, while the aggression looks frightening, DN3 hasn't been badly injured. It is rapidly growing in thermal down, which will help protect it from cold weather and aid nest exploration. Fighting is very hard to watch, but generally rare when one compares the time spent fighting with the time spent laying around the nest and eating.

There are no guarantees, but siblicide is uncommon and DN3 is much stronger than it seems. It often comes back with its own beaking, displaying its own fierceness, or can be seen afterward cuddling up to a former aggressor. We are hopeful it will survive and even thrive as it grows...but again, there are no guarantees with wildlife.

Will you intervene?
Absolutely not. We might consider interfering if the situation were human-caused, but these behaviors are completely normal from an eagle perspective. If we rescued every eaglet we were concerned about, there would be no wild eaglets left to watch. It is very important to keep eaglet behavior in perspective. For the most part, the parents have acted according to our expectations: feeding, interacting, brooding, and in general caring for their family. While the eaglets fight with one another, they spend more time cuddling, eating, and sleeping. While the fighting is upsetting to watchers, none of it is out of line or outside the parameters of normal eagle behavior.

In summary, the worst may happen but we are not giving up on DN3. The next five to seven days will tell us a lot more about its growth. It looks like we are starting to get pinfeathers on DN1, which means its growth will start slowing as feathers take over. While we have never seen this level of aggressive interaction in Decorah, eagles have survived it at other nests. We remain hopeful.

Kay from SOAR also provided some feedback after she was approached about the situation. She agrees that the situation "looks" terrible but that at this developmental stage of the eaglets they don't have a huge amount of strength in their beak.  Right now, those beaks are something like pointy salad tongs. Hopefully all three eaglets will grow well and this will not be a concern for much longer.  

Decorah North: DN1, DN3, and today on the nest

Watchers of the Decorah North Nest got quite a shock today after DN1 went after DN3 with a ferocity we haven't seen at any of the bald eagle nests we watch. It looked liked this:

• Mom North lands on the nest, chasing away a sparrow that was stealing straw from the nest. DN1 and DN2 are huddled together at about 7:00 on the left side of the nest, while DN3 is laying at about 4:00 on the right side of the nest.
• Both eaglets rise to a sitting position.
• DN1 dominates DN2, pecking and pulling on DN2's left wing. DN2 submits and DN1 stops.
• DN1 shoots poop, staying at roughly 7:00..
• DN3 sits up and walks over to the turkey wing at about 3:00, where it appears to be look up at Mom. Keep in mind that the eaglets are walking on their tarsi, not their feet.
• DN1 walks from 7:00 to 3:00 and begins a dominance interaction with DN3. In addition to pecking, DN1 bites and pulls on DN3, who submits. 
• As DN3 submits, it rolls against DN1, scraping DN1 with its talons in what appears to be an accident.
• DN1 resumes the dominance interaction, pulling and biting at DN3's wing and body and flipping DN3 over. DN3 remains in a submissive posture.
• DN1 pulls DN3 up by the 'scruff' of its neck four times
• Mom North, who has been watching the entire interaction, walks over and looks at DN1.
• DN1 ceases the interaction, although we don't know to what degree Mom North interfered with it. Was DN1 ready to quit given the lack of a response from DN3, or did Mom's arrival distract DN1?

Following this event and subsequent battles, the ugly specter of siblicide - one sibling killing another - leapt to everyone's mind. So how common is siblicide in bald eagles? Sources disagree, with some referring to it as relatively common (University of Nebraska, American Eagle Foundation) and others calling it fairly rare (Hornsby). It hasn't yet occurred at any of our cams - not in Decorah (since 2008), not at Decorah North (since 2016), not in Fort St. Vrain (since 2003), and not during the one year we were able to watch Eagle Valley (2013). Our nests have different parenting 'styles', levels of food availability, surrounding environments, and nest invaders, but to date all eaglet fatalities have been caused by hypothermia, predators, and suspected disease. Why did DN1 take after DN3 and (to a lesser degree) DN2 today? We don't know, but as tough as it was to watch, it wasn't especially prolonged and didn't lead to death.

People also worried about a lack of food deliveries to the nest. While we don't know why parents don't pile up the pantree here as they do in Decorah, birds of prey (even young ones) can go a long time between meals. The eaglets remain healthy, alert, and pooping, which tells us that their digestive systems are working just fine. They are moving around the nest, interacting with one another and with parents, and showing interest in their surroundings. I'm sure they would like a meal, but they aren't starving yet and won't be for some time.

Several people referred to Dad and Mom as 'bad' parents. Again, different nests have different parenting styles, but eagles don't divide neatly into human narratives of good and bad behavior. It is a warm day, the young don't need brooding, and either Mom or Dad have been perched nearby much of the day. They are taking care of their young as they see fit, and we know from studies on human-raised birds that young birds have very different needs than young humans.

We've also been asked if we will rescue DN3. Absolutely not. We might consider interfering if the situation were human-caused, but what happened today was completely normal from an eagle perspective. If we rescued every eaglet we were concerned about, there would be no wild eaglets left to watch. It is very important to keep today in perspective. This is one of the first days we have seen a lot of concern about Decorah North. For the most part, the parents have acted according to our expectations: feeding, interacting, brooding, and in general caring for their family. It is also the first day - the first time - we have seen that level of aggressive interaction in one of the bald eagle nests we watch. But none of it was out of line or outside the parameters of normal eagle behavior. If eaglets died after a warm day with just one meal, we wouldn't have eagles to admire and worry about.

In 2011, a follower sent me a lovely watercolor painting of the fledglings at the Decorah nest. It said "You are braver than you believe, stronger than you seem, and smarter than you know." It remains a favorite artwork of mine and I often think of it when I am concerned about the eagles. As anthropomorphic as the painting may be, eagles are truly stronger than we think and often smarter than we know. As much as we love them, they are only their own and what we saw today was simply part of the way they live their lives.

Note: Hornsby follower gzebear compiled observations from 147 nests checked in an 8 year period and found only a 2% incidence of siblicde in hatches of 2, and 3.8% in hatches with 3. Follow the link to read gze's data, references, and additional comments from contributors to the thread. I think it might be time to revisit the whole subject of three-egg versus two-egg clutches. How common is one versus another? Are there regional differences? Is it an inheritable trait? Do we have more three egg clutches than we used to? It's off-topic here, but I subject that interests me quite a bit.

Thoughts on the sub-adult nest-intruder

We had an intruder at the Decorah North Nest on March 13th. While Dad was in the nest incubating the pair's single egg, a roughly four-year old female eagle perched on a limb right next to the nest tree. He responded by covering the egg, softly vocalizing, and ruffling his feathers. The whole encounter lasted about fourteen minutes.



I was intrigued by Dad's soft vocalizations. Why didn't he call a little louder? I went to our panel of eagle experts for an answer. Jim Grier and Brett Mandernack both provided insights into Dad's behavior. Note that neither Jim nor Brett referred to the eagles by title (Dad) or sex. Those additions are my own.

Jim has extensively studied eagles in Ontario and along the Mississippi River Valley. He noted that the density of eagles is getting so high that it might be changing eagle "society" and interactions, or bringing out behaviors that don't occur in lower density [and perhaps limited food-base] situations.While we don't think of the Decorah North Nest as urban, bald eagles along the Mississippi river and its environs are urbanizing as the population density grows. Our initial ideas about the lonely majesty of the bald eagle may have come from observing lower-density populations in a landscape much less impacted by human beings.

Jim explained that the soft vocalizations sound more like typical young- or mate- calls made at close range between young and parents or between mates (as in food-begging by young toward adults or females toward males). He also pointed out the direction of Dad North's gaze. Dad North didn't spend much time looking at the eagle on the branch, almost as it he was familiar with her. To quote:

"Direct" gaze can include both truly direct, focused straight at -- with both eyes and holding the gaze -- or what I call indirect-direct, mostly a side-ways look with occasional looking away and often with raised hackles, all of which is the prelude to an attack if an intruder doesn't back off. When an eagle is presented with a new individual that it doesn't know but which it is not particularly concerned about, it will "study" the new individual, looking directly at it and looking at different parts of its body. Then it will look away briefly, then look back and study it some more, and alternate with increasing lengths of time looking away and shorter periods of gazes ... until it becomes "familiar" with the new individual and then, subsequently, usually only glance occasionally at the individual but mostly seeming to ignore the familiar bird nearby. 

As upset as Dad might look to us, his indirect gaze indicates he might not have been especially concerned with the intruder. There was just one brief threat-bout toward the bird on the branch when he stood up to bring in his wings and change position. Jim suggested that there could be some mutual extra-pair mate attraction going on between Dad and the bird on the branch, creating an interesting mixture of aggressive and compatibility behaviors (soft vocalizing=compatibility, while threat-bout=aggressiveness). While it had never occurred to me that the two eagles might be familiar to one another, Brett also pointed out the possible familiarity of the intruder with the incubating adult. A quick search of documented nest intrusions makes it obvious that this is far from a full-metal response. Here are three aggressive reactions for comparison. Bob was quite fascinated by the second one:

• Night-time raccoon intruder (note very direct response and gaze!):
  https://youtu.be/RWJfTaO-Wz0. 
• Hawk attacks bald eagle on eggs (hawk does not survive - response and gaze are direct!):
  https://youtu.be/2s8anVG-o_I
• Bald eagle attacks owl for trying to take over nest (owl survives and is chased off):
  https://youtu.be/B0eaDz1noRg

Jim went on to provide an alternative interpretation of the interaction.

"An alternative interpretation might be that there is a conflict between opposing behaviors, such as aggression toward the intruder in conflict with, or inhibited by, a higher drive that results in priority on protecting the egg (against either the outsider or accidental damage by the parent itself if it were to move around too much near the egg, as in the behavior of balling of the feet when moving around the egg). All of this is stereotyped action patterning that has been shaped by eons of evolution. The interactions have likely happened many times ancestrally with selection of survival and reproduction producing the optimal solution of trade-offs.

Contributing to all of this is the fact that the perched outsider, younger bird, isn't making moves toward the incubating adult and also not staring at it, but just mostly looking away ... as if they're familiar with each other, or that any serious interaction occurred earlier and have now run their course so we're just seeing the end of it.

All in all, it somewhat reminds of the reduced level (to my Ontario and previous MN experience) of aggressive interactions that I observed among close nests when we were doing the FWS aerial monitoring surveys along the Mississippi River in 2009. We might be seeing a new social order in eagles! (Or, at least new to us, behaviors that are more likely elicited in a new [to us] high-density eagle population [and high productivity of prey base, so food shortage is not a factor].)"

In short, what looked to our eyes like an aggressive interaction was not really very aggressive. Dad didn't challenge the intruder with a direct gaze, loud vocalizations, or an attack. He might be familiar with her (and possibly even attracted to her), or an aggressive response might have been inhibited by a higher priority to protect the egg. The intruder also wasn't aggressive. While she remained near the nest, she didn't try to enter it or engage Dad directly, and Mom displaced but didn't seriously attack her when Mom arrived at the nest. While we think of bald eagles as being non-social on their breeding territories, their interactions with and signaling towards visitors and 'neighbors' may be more complex than we initially thought, especially as the population becomes more dense and interactions become more common.

Many thanks to Jim and Brett for sharing their experience, wisdom, and insight!

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I became curious about eagle-to-eagle interactions after reading Jim and Brett's comments. A few videos:

• Sub-adult chased off by juvenile. This is an aggressive interaction, although no physical contact occurs. Note the loud vocalizing and direct attack: https://youtu.be/vnu0ZEJEAJw
• Adult to sub-adult interaction. Another aggressive interaction. Note that eagle on the nest doesn't stand up to defend until physical contact is imminent, perhaps lending support for Jim's idea that aggressive responses might be inhibited by a higher priority to protect eggs.
  https://youtu.be/9VdEmxLgPGo
• Adult to sub-adult interaction. This is interesting and not especially aggressive, although it includes physical contact. The adult (the video identifies it as male Romeo) is watching something outside the nest. At 3:34, a sub-adult eagle lands in the nest. Romeo flaps at it, but it looks down and away without responding directly to him. For the most part, the two eagles avoid direct eye contact. At 5:31, the sub-adult appears to check out the adults talons. The sub-adult nips at the adult at 5:34 and the adult flies off. https://youtu.be/_7BssmkllQY

We are sure people are curious about whether or not the intruder could be offspring from a previous year. We can't dismiss it, since eagles are philopatric, but we also can't confirm it. Perhaps further research will shed light on the relationships between bald eagles and sub-adult offspring. 

Will the nest trees hold N2B and the Decorah North Nest?

We're getting some concerned questions about the strength of the trees that N2B and especially the Decorah North Nest are sitting in. The North eagles have an extremely large nest by any measure, with an estimated total area of 49 feet and and an estimated weight of around 2100 pounds. Will the trees be able to handle it?

The likelihood of any given tree or branch to break depends on its strength, its size, its shape, its health, and its location, all which affect its capacity to withstand loading by things like wind, ice, and bald eagle nests. In general, thicker trees are stronger than thinner trees, balanced trees are stronger than top or side heavy trees, healthy trees are stronger than sick or dead trees, and trees composed of dense or heavier wood are stronger than trees composed of lighter wood. Location can impact a tree's exposure to lightning and affect how it grows in the presence or absence of light and prevailing winds.

Let's take a look at the strength of five trees: a white oak (the Decorah North Nest), an eastern cottonwood (the Decorah Original Nest), a loblolly pine (bald eagles nest in lobolly pines), a sugar maple, and a shagbark hickory. A few terms before we get started:

• Specific Gravity: Specific gravity measures the density of a material. In general, the higher the specific gravity, the denser and stronger an object is. However, other mechanical properties also influence a tree's strength.
• Modulus of Rupture, or MOR: MOR is a measure of the maximum bending load in a material just before it yields or ruptures. 
• Modulus of Elasticity, or MOE: MOE is a measurement of stiffness that determines a material's deflection from a load.
• Toughness and Hardness: Toughness is the ability of a material to absorb energy and plastically deform without fracturing, while hardness measures the force required to embed an 11.28 mm (.444 in) steel ball into wood to half the ball's diameter. These two characteristics are often inversely correlated in trees: that is, a hard tree can be less tough than a weak tree. However, toughness is often proportional with density, and trees create areas of denser wood where needed. More on that later! 

Wood Species	Specific Gravity*	Bending Strength (MOR)	Stiffness (MOE)	Hardness
	kg/m3	MPa	GPa	lbf
Shagbark Hickory	0.72	139.3	14.90	1,880
Oak, White	0.70	102.3	12.15	1360
Sugar Maple	0.63	109.0	12.62	1450
Loblolly Pine	0.47	88.3	12.30	690
Cottonwood	0.36	58.6	9.45	430

Trees experience dynamic and static loading. Static loads are long-term and don't change quickly. Think of growing limbs or a bald eagle nest. Both of these things place a load on a tree, but they happen slowly and the tree has time to respond to them. Dynamic loads like wind gusts occur immediately and unpredictably, giving a tree no time to respond. Think of N2 - a healthy tree! - snapping during a storm in July 2015.

MOR describes the maximum amount of load that can be placed on a tree before it breaks, while MOE measures its resistance to being deformed elastically. Think of climbing a tree. When you step on a branch, it might bend, but unless you exceed MOE, it will snap back into place once you take your weight off. If you exceed MOE but not MOR, the branch will stay bent once the weight is removed. If you exceed MOR, the branch will break. I hope you are holding on tightly to something else!

Also keep in mind that the measurements for MOR and MOE are mega- and giga- pascals respectively. A pascal is one newton per square metre, or the force needed to accelerate one kilogram of mass at the rate of one metre per second squared. While some of these trees are stronger than others, a lot of force is needed to rupture or displace any of them! A cottonwood has the lowest MOR, but it would still take 58.6 million pascals to break it. That's a lot of static force! The DNN nest might be large, but a sturdy white oak is more than capable of supporting it. If I'm doing the calculation correctly, the weight of a roughly 2000 pound nest converts to 8825 newtons, or .008 MPa, which is nowhere near the weight or force needed to cause static rupture in any of these trees, assuming that they are healthy and the nest is relatively well balanced.

Like people, trees grow and change throughout their lives. Stress effects wood growth or build up: the more stress or compression there is on an area of a tree, the more wood will build up there to counterbalance it, toughening that area of the tree and expanding and pushing back in the opposite direction of the compressive force. You can sometimes see this on the underside of large tree limbs and you can clearly see it in cut wood. Naturalist Bernd Heinrich writes: "In tree limbs the greatest stress occurs in the “hinge” area, at the attachment to the tree. It is here that the growth rings of limbs are thicker on the bottom than on the top. A mechanism of selective wood deposition could help to brace the limbs up laterally." If we sawed through the limbs that supported N2, we might find that they were thicker and had a different pattern of deposition than similar limbs that did not support bald eagle nests. As the tree grows, it responds to the weight of nests and branches by piling on more wood where it is needed - and trees overbuild to begin with!

Of course, limbs and trees do sometimes break. However, bald eagles have a remarkable record of success when it comes to choosing trees.  One study found that, of more than 10,000 bald eagle breeding attempts that were monitored in Virginia since 1962, only one in 850 failed due to tree loss. Nearly half of these losses were due to violent storms that snapped off live trees during the nesting period.  Of the remaining losses, nearly all were in trees that had been dead only one year.

So do eagles somehow know where to place a nest for strength and balance, or is it not possible to begin a nest anywhere that isn't well balanced already? The more questions I ask, the more questions I have! But either way, bald eagles are master builders and the trees we have nests in are more than strong enough to hold them!

Bald Eagle nest in East Central Minnesota

This bald eagle nest is located in a tree about 12 miles from my house. As you can see, it is a big nest in a small tree! Bald eagles have occupied it for roughly four years. During that time, the tree has responded to the weight by deforming and thickening. I suspect this tree will go down some day in a storm, since it is more prone to succumbing to dynamic loading given its top heaviness. However, this photo gives us a great look at just how strong trees really are.

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Things that helped me learn and write about this topic:
This was the most technically difficult subject that I have ever written about. Wood strength and qualities are measured in many different ways, boards are not the same as live trees, and even arborists and horticulturalists think about and measure trees in various ways. Since trees are alive, they also aren't as uniform as, say, a nice pine 2x4. I found all of the articles I read to be fascinating (except the ones I couldn't understand at all) and they helped me to garner a whole new measure of respect and appreciation for the trees that surround us.

• Bernd Heinrich: The Trees in My Forest. This should be required reading in high school. It is available via Google Play.
• Center for Conservation Biology: Eagles Rarely Gamble During the Breeding Season 
• A nice general article on trees: Tremendous Trees
• Static Load Tests in Arboriculture. Note 'Tunnel effects' on page three. After N2 went down, a number of local Decorah people told us that slots and canyons in the valleys around Decorah produced odd wind effects.
• Ken James, University of Melbourne, Australia, Dynamic loading of trees
• American Cutting Edge: Hardness versus Toughness
• The Wood Database. What a cool website! There is more to learn about wood than I ever suspected!
• The Workshop Companion. Great reference site for woodworkers.
• Dr. Kim D. Coder, Warnell School of Forest Resources, University of Georgia, Elastic Limit: The strength properties of living trees

Decorah North Nest

We surveyed a new nest last week. 'Decorah North' is a very large nest in a secluded valley north of Decorah. A few statistics from our trip:

• The nest is 56 feet off the ground 
• It is nine feet long at its longest point
• It is seven feet wide at its widest point
• It has a total area of 49 square feet
• It is about 5.5 feet high
• The eagles began building it in 2013.

Using the method listed here (http://goo.gl/WajKSb), I calculated the nest's weight at roughly 2100 pounds (using 37 pounds per cubic foot for wood weight). Good thing it is located in a sturdy white oak tree!

Kike, a professional photographer and climber, found the following bones in the nest:

• Adult male turkey leg and and ilium
• Turkey polt leg. 
• Fawn leg
• Adult raccoon skull
• Young raccoon skull

We are looking at adding cams to this nest and will keep everyone posted on our progress. Given the nest's size and prey remains, these must be industrious, hungry eagles!

Quick note: The nest is located on private property and can't be seen from the road. We are asking everyone to respect the landowner's privacy (and cattle) and refrain from visiting the nest in person. Thank you.

From above. Kike used his climbing helmet for scale

Another view. More than one person could fit quite comfortably

The nest from the side

The fawn's leg. Note how tightly woven the nest is. 

The nest from above. Kike's shadow helps provide a little scale.