Thursday, April 03, 2014

Eaglet Questions

Some questions from eagle watchers:

What will happen in the first week?
Like humans and other animals, growing nestlings have developmental milestones. The eaglets spend roughly the first week of their life growing. They aren't able to thermoregulate (control their own temperature) yet, so they will spend a lot of time under Mom and Dad, depending on the weather and temperature. We'll see them eat, sleep, scuffle, and grow stronger as they interact with one another. They will probably lose their egg tooth in the first week, but they will not change color until they are a little older.

Here is a slideshow from the first week in 2011. The eaglets are noticeably larger and stronger by the end of the week, although they aren't exploring much yet.

How big were the babies when they hatched?
The quick answer: not very big! Unlike poultry, waterfowl, and even some commonly used falconry birds, it isn't easy to find data on bald eagle hatchling weight. According to the paper 'Physical Development of Nestling Bald Eagle Growth with Special Emphasis on the Timing of Growth Events', written by Gary Bortolotti, the average hatch day weight of bald eagle hatchlings at Besnard Lake in Canada measured 91.5 grams, or 3.2 ounces - roughly the same weight as 18 nickels. I suspect that our little "Ds" would weigh roughly the same amount, although Bald eagles get bigger the farther north one travels: a phenomena known as Bergmann's Rule.

I collected 18 nickels and held them in my hand. It isn't much.

How fast do the eaglets grow?
The quick answer: fast! The maximum nestling weight gain for Bald eagles appears to be the greatest in North America. From the same paper by Mr. Bortolotti: Male hatchlings had an average growth rate of 102 grams (roughly 3.5 ounces) per day, while females averaged 130 grams (roughly 4 ounces). However, their growth occurs along a curve, not a straight line, with the highest percentage of weight gain per day occurring in the first 30 days.

Math Warning! Regular readers might recall that I used the fibonacci ratio of 61.8% to try to determine nest weight. This is a number that turns up all the time in nature, especially in spiral shapes. Those of you that really love math will be pleased to hear about the Gompertz function: a model that describes a time series where growth is slowest at the beginning and end of the series. Like the fibonacci ratio, this turns up all over the place: eaglets, tumors, population curves, and market saturation can all be modeled with the Gompertz function. This chart shows the pattern of the eaglets' growth as a curve from somewhat line-like through very curve-like to almost line-like again. This special kind of curve is called an asymptote: a kind of curve who's curvature approaches zero as it tends towards infinity. Put simply, it becomes more line-like and less curve-like the longer it goes. Note how steep the curve is between 0 and 1 when considered as a percentage of the asymptote. That's a fast-growing animal, and most of its weight gain, relative to its overall body weight, occurs early on.

This chart was reprinted from The Wilson Bulletin 96: 527 from an article published by G. R. Bartolotti (1984) with the written permission of the Wilson Ornithological Society.

Of course, not all body parts follow a nice neat curve. Again, back to Bortolotti's paper: the legs of the nestlings he studied appear to have reached full size about halfway through the nesting period, but the flight feathers and bill didn't attain mature size (or, in the case of the feathers, mature juvenile size) until after the eaglets left the nest. Similarly, their flight muscles don't fully develop until after they've started flying. Like so much else in nature, what looks like a simple question turns out to be surprisingly complicated. In general though, fast works well to describe eaglet growth.

What about Cain and Abel Syndrome?
A lot of people are worried about Cain and Abel Syndrome - the harassing or killing of a smaller, weaker sibling by a bigger, stronger sibling. In the case of the eagle nest, the first born eagle is most likely the largest and strongest. As those crazy growth curves make clear, it might have a substantial advantage in size and weight over its siblings.

Having said that, this nest has a very good history of producing young. While the siblings still harass one another, the area's abundant food supply helps mitigate the affect of Cain and Abel Syndrome. I don't like to make predictions about wildlife, but we haven't seen one die as the result of Cain and Abel Syndrome yet. The harassment will slow as sibling growth achieves parity: not unlike some human families I can think of!

This razzle video from 2012 shows sibling competition.

What is a Crop?
The crop is muscular pouch in the esophagus that stores food and regulates its flow through the digestive system. A distended crop can look quite alarming, but it just means the babies are digesting a recent meal. When our moderators talk about crops, they are usually pointing out that the babies are full or answering a worried question ("What is that?!"). That bulgy-looking thing at the top of the babies' chests? That's a full crop.

This raptorfan video from 2012 shows a feeding:

Where does the word pip come from?
This is one of the things I like about our watchers - they always come up with new questions that I would never have thought of asking. Pip has a number of meanings, but it appears that this useage might come from 'pip' in the sense of 'spots found on dice or playing cards' and is derived from the old English word peep. I recommend googling this one - it's a fun word to explore.

Things that helped me learn and write about this subject:


Lisa Harris said...

thank you for all the info and the videos, i have added this to my favorites.

Dawn Brown said...

about the Decorah eagles-is there something wrong with D19? it seems to not be eating and D18 seems to knock it down at feeding time. Is this what I'm seeing or am I confused? We have been watch pretty constant for the last 4 days

Unknown said...

I just saw the nest with no third egg..... has that egg hatched or been ditched????

Gwen Sindel said...

What happens to the empty egg shells?

mjnancy said...

In some of the activity descriptions I see the letters PS. What does that stand for?

mjnancy said...
This comment has been removed by the author.
IleanaDU said...

Is there supposed to be a link here?

"This razzle video from 2012 shows sibling competition."