Re: Ivory-bills by the numbers

Cyberthrush put up a post attempting to use math to argue that it's unlikely that the Ivory-bill could have disappeared "in a mere 60 years". I admire the effort, but I think he's vastly overestimating the bird's nesting success and vastly underestimating the bird's natural mortality.

On nesting success: Cyberthrush assumes 6 nesting attempts resulting in 10 fledglings; in Tanner's book, 6 real-life Singer Tract nesting attempts resulted in a total of only 3 fledglings.

On natural mortality: Cyberthrush has the average fledgling living to 10 years of age. I don't doubt that a lucky Ivory-bill could live to be 10, but assuming that the average bird lived to be 10 is a huge stretch.

It's a dangerous world out there, especially if you're a bird. We don't have data for Ivory-bill natural mortality, but consider this "Birds of North America" data for Pileateds:
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Of 31 radio-tagged adults followed for 2 yr in ne. Oregon, 55% survived (ELB). In w. Washington, 43% of radio-tagged adults survived 1 yr (K. Aubry and C. Raley pers. comm.).
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In other words, roughly half the Pileated adults died off every year or two. Under those conditions, a fledgling would be lucky to live to 3 or 4 years of age.

Also note that Tanner said (page 83) that "...it is possible that the greatest factor reducing the rate of reproduction of the Ivory-bill is the failure of some birds to nest".

Bottom line: The data is incomplete, but I'm not convinced that the Singer Tract birds were breeding at replacement value. I think it's possible that the Ivory-bill would have become extinct even if the Singer Tract had been left undisturbed.