Sunday, June 27, 2010
1:08 AM | Posted by Sweep Commander | Edit Post
There's a small mystery on Alcatraz right now and only our ravens know what's really going on.
The ravens bred in 2010, but only one branchling left the nest alive. The adults watched over their young one closely and killed an adult western gull, apparently for showing a little too much interest in their pride and joy.
Then, suddenly, we went from three observable ravens to two. It seemed that the juvenile was gone. As new ravens will stay with their parents for a period of months, successful fledging and departure seemed unlikely. The speculation was of its possible demise.
As usual, the ravens had us fooled. The juvenile was still very much with us, and hiding right before our eyes. She or he vocalized softly and hoarsely while displaying submissive crouching and begging behavior typical of a juvenile or a subordinate individual in a larger group but unlikely behavior for for a breeding adult.
One mystery was solved. One of our visible ravens was the juvenile. The other was the male, discernible by its size and its trademark "fur pants" dominance display. So where is our breeding female? She's been unobserved for quite some time now.
I have a crazy hypothesis: she may be incubating a second clutch of eggs.
This would be more than unusual. It's almost unprecedented. Ravens are not double brooders, though given enough time, they will attempt another clutch if their first brood of chicks and eggs fails completely. Double brooding by ravens is so uncommon that it's only been documented in one single instance, during breeding season 2001, on the campus of UC Irvine.
My suggestion is just a hunch, but it has the following considerations in its favor:
First, there's time for a second brood. The ravens nested early this year. In the single documented case of double brooding, enough time and plentiful food resources were cited as possible factors enabling the behavior.
As for food, with a home territory like Alcatraz, with so much ecology and so much trash at hand, food wouldn't seem to be a problem.
Second, the natural resources arm of the Golden Gate National Recreation Area has visited the ravens' nest multiple times in 2010 and damage to the nest may have occurred. Just as the number of visible ravens dropped from three to two, they were observed carrying nest building materials to their tree, perhaps repairing damage their nest structures incurred. Bernd Heinrich, noted raven expert and author of the great book Mind of the Raven, observed that the ravens' cue to mate is not a ready female, but rather a finished nest.
Damaging the ravens' nesting structures may have inadvertently put them on a path towards a second clutch.
The third consideration is that while double brooding has never been documented except in the one paper I linked to, the paper's authors complain that not enough observations of this type have been attempted, especially on the divergent California clade, to which our ravens belong.
The surprising genetic differences between ravens of the American southwest and those found in all other contexts will be addressed in an upcoming post laying out what little we know about Alcatraz's raven pair.
The last consideration I'd offer in favor of the double brooding hypothesis is that Bernd Heinrich also says that if a raven minding a home territory loses its mate, he or she will find a replacement quickly, often within a day. That hasn't happened, though another plausible explanation for that might be that it's simply too late for courting and pair bonding this year, though I don't recall Heinrich indicating that this behavior is variable across the seasons.
This, combined with my gut feeling that the outright death of the female seems pretty unlikely, gives me the sense that our ravens are up to something once again.
This double brooding suggestion may be absolutely wrong and I realize that it sounds crazy. My hunches regarding birds are almost universally mistaken but if I'm right in this case, I'd like to be able to say I saw it coming.
The island loves to make a liar out of me. No sooner did I write a post entitled "A Coda on the Falcons of Alcatraz" than did the male peregrine decide to show himself on a consistent, daily basis. For now, the kid is back:
Now all he needs is a female companion to really give Alcatraz's ruling authoritarian corvidocracy a run for its money and I wouldn't miss it for the world.
Also as I disembarked onto the Alcatraz dock on Thursday, I was rendered speechless. I'd heard about ducklings on Alcatraz but neither BourbonHawk nor I had ever seen any. But there she was, a duck mother and her eight chicks, soon to be food for the predators that inhabit the island and the bay.
Sometimes words aren't good enough, so I slapped BourbonHawk on the arm and pointed them out to her, this late in the year and bobbing far too close to the machinery mounted to the rear of the boat we'd just disembarked from. We had a few jokes about these witless ducks being sucked up to their doom, but fortunately that didn't happen.
At any rate, I know they're gonners. My only hope is that they get taken by some novel predator like a seal, a porpoise or a sea lion. I'm getting a bit tired of all of this gull and raven brutality.
Here's your update on Ryan T. Gosling, the honking miracle. He's almost there. At this late date, it's a bit difficult to tell which one of the three he is, but I usually assume that the two proudest looking geese are his parents, and he's just the other one.
Brandt's cormorants! They're back indeed. Here's a picture from last year's disastrous lack of a nesting season:
And a picture from our last very good year, 2008:
And here's a picture from last week:
Progress is clearly being made and there are nests on the ground, but they may be dummy nests. These cormorants have built them multiple times now, only to fail to lay eggs and to retreat from them at any disturbance.
Still, the cormorants are having a resurgent year. They may not yet be successfully nesting on their old colonial grounds, but they're nesting less visibly on cliffs all around the island. I don't envy the wildlife biologists in their task to count them and measure their output in such treacherous places.
Lastly, I authored a post last night on a western gull predating a snowy egret chick. I know this post has been a long one, but check that one out too, especially if you don't mind a gruesome image to go along with it.
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- A Coda on the Falcons of Alcatraz
- Nesting season update on Alcatraz's newest western...
- New post!
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- Mystery solved...
- It's a mystery!
- Juvenile raven vanishes, breeding parents set to w...
- ▼ June (9)