[SBB] Golden Eagle behavior??

[Ruth Troetschler <[[email protected]]>]

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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