Re: [SBB] Golden Eagle behavior??

[Bill Bousman <[[email protected]]>]

Folks:

I have some comments on the Jasper Ridge Golden Eagles.

I have not read the Birds of North America 
account, but anyone who is interested can find 
this at the Falconer Library at Stanford.  A 
careful reading of this account will show, I 
expect, that although there are general behaviors 
that can be described, there are many exceptions 
and great gaps in our knowledge still remain.

If you are interested in detailed studies from 
California, then two that were published in the Condor will be illuminating:

Carnie, S. K.  1954.  Food habits of nesting 
Golden Eagles in the Coast Ranges of California.  Condor 56:3–12.

Dixon, J. B.  1937.  The Golden Eagle in San 
Diego County, California.  Condor 39:49–56.

These can be accessed from the SORA website.

Dixon's studies showed that Golden Eagle 
territories are more or less permanent.  When an 
adult dies, it is quickly replaced by a bird 
within the floating population.  I don't recall 
that Dixon ever encountered an immature bird 
nesting, much less two, but perhaps a rereading of his paper would be helpful.

Carnie found most birds in the Diablo Range 
nesting in foothill pines, which matches our 
atlas experience.  My guess is that most pairs in 
the Santa Cruz Mountains nest in Douglas 
firs.  Milton Ray collected at least four sets of 
eggs from various locations in San Mateo County, 
although my records do not include mention of the 
trees used.  Conventional wisdom holds that there 
are about five pairs in the Santa Cruz Mountains 
and, if Dixon's conclusions apply to this area, 
their territories are constant.  The one pair 
that is regularly studied, as mentioned by Ann 
Verdi, nests in a transmission tower rather than 
a tree near Calero Reservoir.  This pair has 
nested here with reasonable success for at least 
the last 15+ years.  Many pairs studied elsewhere 
in California have shown greater longevity (in 
the sense that a "pair" is defined by the 
territory, not by the individual birds, which may 
not survive for such periods).

The observation of two birds at Jasper Ridge 
apparently was brief and there are probably 
numerous possible explanations for the 
observation.  The birds may or may not have been 
related.  Their encounter, as viewed by the 
observers, may have been a unique, one-time event 
or a regular occurrence.  I don't think that any 
conclusion can be established by such brief 
observations except how exciting it is to be in 
the presence of these great raptors.

Bill


At 11:14 AM 8/21/2006, Ruth Troetschler wrote:
>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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