Friday, June 25, 2010
Baby birds on Alcatraz are risky ventures indeed
Warning: The following post contains a graphic image of a dead bird
Not every infant bird faces survival prospects of less than one percent, but as Darwin noted, there are always too many young for the resources at hand and inevitably attrition and competition do the dirty work of eliminating the excess.
In the world of humanity, the west has infant mortality figured out. In the United States the rate has dropped to just .64%. The highest rates of infant mortality in the United States are found in Washington DC and Mississippi, with 1.41% and 1.074% respectively. The lowest mortality rates are enjoyed by Minnesota and Massachusetts, with .478% and .489%.
Angola has the highest infant mortality rate in the world, where tragically, 18% don't make it to their first birthday.
As with human beings, mortality rates among many young birds on Alcatraz are tied to environmental factors, like the availability of food and fresh water. Our baby birds also have to deal with the presence of the hostile predators which place an additional strain on their prospects.
Let's talk about a curious difficulty faced recently by one of our most successful breeding species, the snowy egrets.
I've seen western gulls hovering low over the thicket where the egrets make their nests and fledge their young:
But I'd never seen one dip down and actually grab a vulnerable chick from an exposed branch. But this last week, that's exactly what happened and this was the unfortunate aftermath:
The victorious western gull was defensive of his prize. Though he obviously destroyed the head of poor chick, he failed to open the body cavity, and his best apparent efforts were gentle tugs on the dispatched egret's wing. As far as anyone can tell, the effort yielded no food.
It reminds me of the time that BourbonHawk, LadyBird and I witnessed a western gull take a tiny gosling in Golden Gate Park. The gull lost interest and several gulls half heartedly fought over the gosling's body, before an immature individual finally and nonchalantly swallowed it.
The egrets have it much better than our black crowned night heron eggs and chicks that our ravens mercilessly predate, but they're still vulnerable to this pointless and opportunistic predation.
Here's a live, healthy snowy egret, on the road to fledging:
The vocalization combined with the display has become known to Maganrord as "The Dinosaur Dance," for obvious reasons. So rad.
And here are their curious shins:
They're almost scaly.
Check back with us soon. We have so much to report, on subjects as diverse as ravens, falcons, and white crowned sparrows!
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