March 18, 2006

Eaglet Gender

familyNest Update: First, we want to remind our cam watchers about the website maintenance that will occur on early Monday morning. Moving our website from one server to another will help our web host more than it will our website, in that cam watchers will probably not notice any faster service on the cams. But since our site is drawing a large amount of traffic, our web host asked if they could move us to a more powerful web server.

Also, some excellent news on the cam front: We have been told that, weather permitting, Craig Koppie -- the US Fish and Wildlife Service endangered species specialist -- will likely climb the cam tree between March 22-24, but most likely on March 23. He will check the family and the nest structure, and while he's there, recenter our camera. So keep your fingers crossed that he's able to make the climb.

As for the eaglets, they will be one month old next week, and our latest gallery update has photos that indicate our chicks are beginning to discover their wings. This is an exciting event, as it starts their transformation into mighty birds of prey. As the eaglets grow bigger, we'll see more of this flapping behavior, and once they have their feathers, we'll see them take short hop flights around the nest. Then they'll begin branching -- or sitting in the branches of their nest tree -- and not long after that they'll have their first flight.

And speaking of the eaglets, a cam watcher recently asked about the eaglets' weight. I would guess that they're between 3.5 lbs to 5.5 lbs at this point. The oldest is certainly heavier than the youngest since it is fed more and since it was born two days earlier.

On a final nest topic, one of our cam watchers pointed out a good photo from the Llano, Texas eagle website showing how the parent sits above the nest to watch the chicks. You can see the photo here; this is a good example of what our eagle parents do when we can't see them in the cam image. Also notice that the eaglet in the shot is making a hop flight in the nest.

You can see all the photos of the Llano, Texas eagle nest by visiting their home page. They have a remarkable collection of images that chart the eaglet's growth from chick to fledgling.


Eaglet Gender

At this point we naturally wonder about the sex of the cam eaglets. Determining sex is hard when chicks are young because the order of their birth can determine how big they look in the first month or so of their lives. As they get closer to full size, it might be easier to tell.

But is there a sex bias in newborn eaglets? Does one gender dominate at birth? In the book The Bald Eagle: Haunts and Habits of a Wilderness Monarch , Canadian eagle biologist Dr. Gary Bortolotti offers evidence of a sex bias based on observations that he collected while studying 37 eaglet broods around Besnard Lake in Saskatchewan, Canada. He reports, "Even though there was an overall nestling sex ratio of one to one, the order in which the sexes hatched was not random; 63% of first-hatched eggs were females, and 68% of second-hatched eggs were males. A sex bias in hatching sequence has only recently been discovered in birds."

Bortolotti speculates that this arrangement might benefit the family because a male eaglet develops faster than a female eaglet, and females being born first would help even out competition in the nest.

Last year we had three eaglets at the Eagle Cam nest. As those who followed last year's cam know, Craig Koppie climbed the cam tree to remove the middle eaglet so it could be taken to Vermont, where they are attempting to reestablish a breeding population of bald eagles. After Craig left the nest, he reported that the oldest and youngest eaglets appeared to be males. We now know that the middle eaglet (the one that went to Vermont) was in fact a female, as she was measured and tested after she was removed from the nest.

If Craig's observation was accurate, then according to Dr. Bortolotti, that means last year's Eagle Cam nest was unusual in that a male was born first and a female second. We'll never know for sure because our males were never tested, but it's still interesting to think about.

This also brings up the question of how to tell the difference between an adult male and female bald eagle. First, it's important to note that bald eagles in different parts of North America are different in size. Eagles that live in Alaska are larger than their fellow eagles in southern areas such as Florida.

call eagleBut for eagles that live in the same range, females are about 1/3 larger than the males in body size. Also, biologist Dr. Mark Stalmaster discovered that two size measurements -- beak depth and hallux (toe claw) length -- can be used to differentiate between the sexes because the female's beak and claws are bigger.

Biologists who have actually handled bald eagles also report that they have their own way of judging gender. Jim Watson, who is an eagle expert for the Washington Department of Fish and Wildlife, has said that he can tell gender by the eagle's temperament: females are docile, while males are very aggressive in hand. He also reports that he can tell the sex of an eagle by its call: the female has a lower pitched call, while the male's fluting call is more like a scream. You can hear an eagle call here (143KB MP3 file).


Until next time,
Lisa - webmaster
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Posted by Webmaster at March 18, 2006 11:30 AM