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IVORY-BILLS  LiVE???!  ...

=> THE blog devoted to news and commentary on the most iconic bird in American ornithology, the Ivory-billed Woodpecker (IBWO)... and... sometimes other schtuff.
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Thursday, September 29, 2005

 

-- Ivory-bills by the Numbers --

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Okay all you math geeks (and anyone else) follow me on this:

1. Let's start with a population of 16 Ivory-bills.

2. Let's say 25% of all IBWOs were killed by hunters/collectors before they could breed -- MUCH higher % than most would propose -- but this gets our starting population down to 12 immediately.

3. Let's say the average lifespan of an Ivory-bill is 10 yrs. -- at the LOW end of most estimates.

4. Let's further presume they can't breed for their first 2 years or their last 2 years -- leaving them 6 breeding years. And let's further say, for the heck of it and for reasons unknown, they routinely fail one of those years, leaving us with just 5 breeding yrs. in a lifetime for the average pair (probably way short of the truth, but whatever).

5. IBWOs were known to generally lay 3-5 eggs per nest, though often for reasons not fully understood, they only averaged raising 2 chicks.

6. So with 5 breeding years a typical pair might produce as few as 10 offspring in their entire lives. Let's say (being further conservative) that HALF of these, for whatever reasons, don't make it to adulthood. Now a pair of IBWO produces just 5 offspring in their lives.

7. We started with 16 IBWOs, reduced to 12 birds or 6 pair (for the sake of argument). These 6 pair, selecting very conservative values, could've easily produced 30 offspring in their lives (if they lived longer, bred more years, raised a higher percentage of their eggs, or got shot less, the number only goes UP!). Yet another HALF of those offspring would somehow have to fail to live/breed before you would be below replacement value for the original 16 birds. And for the species to go extinct of course you MUST repeatedly get below replacement value. I think this is difficult (not impossible) to do using reasonable guesstimates. The key of course is the IBWO's longevity and the fact that it was not often predated by anything other than Man (small songbirds that only live 3-5 years, and are easily predated or nest parasitized, can suffer extinction much more easily). Others will argue that habitat destruction caused near complete termination of IBWO breeding (in fact it's virtually the ONLY argument they can make), for if each pair merely produced 3 young in their lives they had more than (by 50%) replaced themselves. It's difficult to imagine them NOT reaching this figure in 10 years, let alone the 15-20 yrs. that most folks estimate for their longevity.
In short, given the end of hunting pressure on this bird, the increase in habitat ever since the 50s or late 40s, and every creature's normal 'will to live and breed', I think it is EASY to account for any population remaining in Tanner's time still being with us today (there is NOTHING EXTRAORDINARY about that AT ALL!). Yes, it is possible that if Tanner's low-ball estimate of 24 birds remaining in the 40's was precise the species just may have run into a "genetic bottleneck" (as some argue) that could have doomed them, but this is not inevitable, and if there were actually 50 to 200 birds left as many believe, I think the above math makes it unlikely the species could have disappeared in a mere 60 years. But hey, you do your own math...
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Comments:
I respectfully disagree with some of your assumptions.
I've posted a response
here .
 
The forces that were the cause of the precipitous decline in Ivory-bill populations were still around after 1940. (The main factor is usually thought to be loss of good habitat. Ivory-bill habitat has only recently gotten better.) So, mathmatically, one would expect the population to continue to fall steadily at a similar rate. Therefore, if you started with 24 birds in the mid-40s (some would say you were starting with one)Ivory-bills would soon be extinct.
 
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