Saturday, May 8, 2010
7:35 PM | Posted by Sweep Commander | Edit Post
I don't blame you guys for not reading or responding to my last post. I can't help getting
a little way too nerdy with it sometimes.
Though we've been slow with updates lately, every day this week we'll be hitting you with real birding news from Alcatraz that you might actually be interested to read, complete with the rad photos and videos you've become accustomed to.
Tonight, we'll start with the island's common raven pair as they've been behaving strangely.
We at Maganrord have it from a reliable source that two raven chicks have hatched, and so it goes for the park's efforts to hinder them through the oiling of their eggs and the dismantling of their nests. The ravens also produced young last year despite the park's best efforts to thwart them.
Now, oddly enough, the ravens are not visibly gathering food for their young but are gathering fresh nesting materials instead.
Did their two chicks fail? Are they rebuilding the nesting structures that were recently dismantled? Will they lay a second clutch of eggs as they apparently did last year after their first was oiled by the park service? At the moment, we don't have much to go on.
Two weeks ago, BourbonHawk and I found the bloodied head of a tiny bird, as well as scattered pale green eggshell fragments along the side of the cellhouse.
It's typical predation by our raven pair. BourbonHawk took the picture. The male raven was hopping around above us, looking down from the roof of the cellhouse as I made this discovery. Both the snowy egrets and the black crowned night herons lay eggs of this color.
The park gives the ravens' predation of the black crowned night herons' eggs and young as the primary justification of its raven management policies. These include oiling their eggs and dismantling their nesting structures.
The black crowned night herons themselves are known for predating the eggs and young of other vulnerable nesting species. Though the irony of their own young being predated is rich, and though our two common ravens are exceptionally huge, there is apparently no truth to the rumors that our common ravens are on a mission from God to exact a penance from black crowned night herons at large. It's just what birds do.
We at Maganrord have to confess the following: observing the ravens is perhaps the most rewarding birding to be had on Alcatraz. Aside from Canada geese and brown pelicans, these are the largest birds we have. These creatures are majestic, imaginative, fierce and intelligent. Rather than being perceived as invasive and dangerous pests to be managed, we would have them observed and appreciated as the compliment to Alcatraz's ecological riches that they very truly are.
Earlier in the year, we observed them tearing up the anti-erosion netting that protects the western beds of Persian carpet flowers. Though the ravens are mischievous, they aren't acting solely to cause headaches for the Garden Conservancy volunteers that care for our gardens. They're out to line their nest with the netting, which has the tactile quality of a burlap sack or twine. Ravens are known to line their nests with moss or fur, but the netting protecting the Persian carpet is plentiful and retains its softness over time.
Note that the confused gull in the upper right corner watches the raven and then sheepishly picks up strands of the netting as if to ask, "What's this craziness for? Can I eat this?" Eventually the gull shows its smaller intellect, understanding only the inedible nature of what it's got. It drops its cargo and goes back to work doing mundane gull things.
Are these ravens really bad news for the black crowned night herons? Absolutely. But the night herons' prospects are hardly enhanced by sharing a small breeding colony with over two thousand large gulls. In fact, though we are absolutely amateurs and our opinions on these matters should be suspect, our review of the available studies indicates that the presence and reproductive success of black crowned night herons is hampered primarily by the gull colony and even inversely correlated to its size. These studies name gulls as competitors to herons for nesting sites and food resources as well as significant predators of the herons' nests and young. We at Maganrord have been joking for some time that if you want to help the black crowned night herons breed on Alcatraz your management policy should be to downsize the gull colony, perhaps by half. The studies available suggest gull management, but we don't anticipate that happening any time soon.
By contrast, the ravens seem to be justifiable targets, as according to some, they are expanding out of their prehuman range.
Other studies indicate that black crowned night heron reproductive success per nest at Alcatraz has in recent years been far lower than at other San Francisco Bay island colonies. Paradoxically, more productive night heron colonies have collapsed and disappeared while the low-output night heron presence on Alcatraz has grown in size.
North bay black crowned night herons may have preferred Alcatraz in recent years, but they have had a number of difficult years here. Encouraging them to settle elsewhere in the bay may be worthwhile.
It's also worth noting that while we're engaging in management policies to limit the spread of the common raven and lessen its impact on the local black crowned night heron population, this heron is one of the most successful species in the world, breeding on five or perhaps six continents and needing no help (other than perhaps feathered hat enthusiasts not killing them for their plumes) to thrive.
On the subject of the raven management policies there is just too much to say and it sounds like the subject of a long and nerdy future post.
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- ▼ May (13)