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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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"....The truth is out there."

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Monday, July 07, 2008

 

-- Population Stasis --

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Summer re-run --- a repeat of thoughts covered here before:

The lack of a definitive Ivory-bill photo from somewhere by now is troubling but points no more to Ivory-bill extinction than it does simply to Ivory-bill scarcity. (Only the ubiquity of photography in today's society and field studies, a relatively new phenomena, makes it seem more convincing to many.)

Assuming IBWO scarcity then, the more difficult issue to grapple with is how such a scarce species could even manage to hang on throughout decades. What such long-term rarity implies is the existence of small populations in near enough proximity to one another that dispersing offspring could relocate and find mates (and as a powerful flyer the 'proximity' needn't be as close for Ivory-bills as for many other species). Creatures in low densities with limited habitat and resources can reach population stasis, or steady-state numbers, that may be successfully maintained over lengthy periods with fair ease. For Ivory-bills, all that is required are a few bottomland or riverine corridors connecting small populations and the birds might maintain themselves at low numbers over long periods, very occasionally being seen and heard, and difficult to get a clear irrefutable photo of --- not unlike, need I say it, the situation at hand (...or at least as plausible an explanation as writing off 100's of claims over years to 'mistakes').

Make some conservative assumptions: Ivory-bill life-expectancy of 10 years (most say it is longer); no breeding during the first 3 or last 3 years of their lives, leaving just 4 breeding years (again, wholly unlikely); out of 3-5 eggs laid, on average only 1 hatchling survives.
Given these assumptions, a single pair of ivory-bills, on average, produces 4 offspring in their lifetime (i.e., 4 birds produced to replace 2) --- if half those birds go on to find mates and continue the process, stasis is maintained. To the degree that some of these conservative numbers increase, IBWO population may even increase slowly over time.
If habitat itself is in a somewhat steady-state situation (some good habitat growing, while other habitat diminishing), not much may change; only to the degree that habitat is improving for the species, might population markedly grow and encounters with the species increase to a point of easier detection and confirmation.
Of course the point of organized, large-scale searches is to increase the number and likelihood of such encounters. Whether they have done so or not is a whole 'nuther debate.

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