Re: [SBB] Golden Eagle behavior??

[[[email protected]]]

All:
Siblicide is not mandatory in Golden Eagles - it is in some species. 
It occurs most often when food is scarce.  My first experience with
Golden Eagles was a cliff nest with three eaglets near fledging. 
Conditions must have been very good indeed.

Charles Coston

---- Original Message ----
From: [[email protected]]
To: [[email protected]]
Subject: [SBB] Golden Eagle behavior??
Date: Mon, 21 Aug 2006 11:14:45 -0700

>Below is a copy of a series of emails from Jasper Ridge Chat, 
>forwarded with permission.  What do you think?  (Read from the bottom
>
>up)
>
>Ruth Troetschler
>
>
>A very nice summary. I have some additional thoughts. There is an 
>essay in the Birder's Handbook, page 307, on brood reduction. It says
>
>that eagles and boobies exhibit "obligate siblicide" and the larger 
>chick invariably kills the smaller sibling even if there is abundant 
>food. So, I am wondering if the pair we are seeing are a relatively 
>rare occurrence here for golden eagles. I have observed quite a few 
>immature and subadult golden eagles in our area and have never seen a
>
>pair of immatures or subadults together. One further thought is based
>
>on the very nice photo Toni got. The white wing patch on the upper 
>wing is very clearly defined. I know that this is not a reliable 
>indicator, but the wing patches of subadults that I have see tend to 
>be less well defined in white/brown contrast. Therefore, my guess is 
>that this pair are birds hatched this year. I would doubt if they are
>
>together as a potential early breeding pair. My guess is that they 
>are too young. I would be interested in views of our more experienced
>
>birders on the list. Thanks again. Bill
>
>At 10:33 AM 8/15/2006, John Kriewall wrote:
>I saw two juvenile golden eagles Monday, 8/14, about 10:30.  I'm 
>guessing this is the JR twosome Toni reported 4-5 days ago. The 
>twosome I saw were gliding over Ladera Oaks Swim and Tennis Club on 
>Alpine Rd.  They were high and moving in a west to east direction and
>
>were characterized by being dark overall with white underside wing 
>patches; the patches looked almost circular from the ground.  Both 
>birds were gliding with wings arched a bit and tails unfanned, so the
>
>white tail banded by dark that Toni and her group saw was not 
>visible.  Perhaps they were heading for the dish area and a ground 
>squirrel lunch.
>
>When I read Toni's post, I started wondering why and how long the 
>juveniles "hang" together.  I assumed that such large animal would 
>need a good-sized catch to sustain itself and would be loathe to 
>share it.  My assumption was based on my knowledge that most bird 
>species, especially where there is asynchronous laying, hatching, and
>
>size in the nest, will frequently will attack/push from the nest the 
>smallest sibling.  I assumed the competition among siblings
>continued.
>
>My sources are Erlich's The Birding Handbook, The Sibley Guide to 
>Bird Life and Behavior, Field Guide to Birds of North America 
>(National Geographic), and The Sibley Guide to Birds.  Here we go:
>1. Two-three nests (occasionally more) are used in alternative years;
>
>they are used perennially so they become "very large".  Since the 
>oldest known Golden Eagle in the wild lived to 38 years and one in 
>captivity to 50 years, one can see the potential for remodeling 
>activity.
>
>2. Two eggs are the typical number laid, in a range of 1-4. 
>Incubation time is 43-45 days(!).
>
>3. The female, which is larger, is the primary incubator, perhaps
>because
>
>a) the male's smaller size results in less effective incubation, b) 
>the energy required in laying large eggs (close to 3" long), or c) 
>the large female is better able to defend the nest.  (However, 
>"reverse" size dimorphism as seen in the world of raptors is not 
>consistent.  Certain owls do not exhibit reverse size dimorphism. 
>The most consistent correlation is those species most likely to prey 
>on other birds exhibit the most size dimorphism.  One explanation 
>could be the size difference encourages seeking food of different 
>sizes and in different food niches.) I don't think this phenomenon is
>
>operant among Golden Eagles.
>
>4. The adults are monogamous.
>
>5. The chicks are semialtricial, so that leads to a feeding time of 
>66- 75(!) days.  The siblings do, indeed, engage in sibling-icide 
>when young. Perhaps early "pruning" of the nest population relieves 
>the murderous hunger pangs and a different relationship develops.
>
>6. In case you're worried about the fouling of the nests, the adults 
>do not defecate in the nests, and the chicks, when moved, point their
>
>heads toward the center of the nest, back to its edge, and defecate 
>over the side.  This results in a heavy whitewash on the surrounding 
>area; if the nest is on a cliff, that whitewash can be visible at a 
>distance, as you can imagine.
>
>7. Prey includes especially jackrabbits but also birds, reptiles, 
>insects, and (I assume) ground squirrels.  If mammals are scarce, 
>carrion is included. (As an aside, pecking order at a carrion site, 
>puts the Golden Eagle ahead of the California Condor; there's a sight
>
>I'd love to see.
>
>8. Subadult birds occasionally breed (one possible answer to my 
>question). These birds routinely are observed hunting solo or in 
>pairs.  Occasionally, they roost communally in winter when prey 
>densities are high.  In most of West, territories are occupied 
>year-around. (Although we know that Golden Eagles do participate in 
>the Golden Gate migration each year.
>
>Conclusions:
>1. If Golden Eagles nest in JR, they may not be seen every year, but 
>they will return to that site in the future.
>
>2. Given the re-use of nests, the likely amount of whitewash, and the
>
>nest size, someone in the JR community must have seen a nest site in 
>JR if it exists.
>
>3. The competition for prey must not be so severe, at least this 
>season, as to limit the willingness of juveniles to hunt together.
>
>4. Or, this may be a bonded pair (I could not detect any size 
>difference but I never have with Golden Eagles I've seen in the past.
>
>Don't ask if they are incestuous!
>
>5. And based on my unscientific assessment of the number of ground 
>squirrels (a population explosion?) in the hills around the Dish, 
>communal roosting may occur this Winter.
>
>I hope this has been educational. It has been for me.
>
>John
>
>----- Original Message----- Subject: golden eagles
>
>Hi all - myself, Ann Lambrecht, John Rawlings and Carol Zabel saw two
>
>juvenile golden eagles today. We were coming down trail E the 
>chaparral fire road, toward trail 13 (causeway) and they flew right 
>above us, about 3 feet away. They may have been sitting on the 
>vegetation on the ridge above us and we startled them. They were very
>
>dark, very large and had the white tail band very visible while they 
>were flying. We definitely agree after looking in the bird books that
>
>that is what we saw. We were very impressed and are not birders 
>(except Carol), but more like that would get me to look up a lot 
>more. Hope you are all having a great summer. Toni
>- ++**==--++**==--++**==--
>
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