strating N assumptions
Having read through the paper – I believe this to be the salient point: these species can persist…provided they maintain ≥ 1.1. recruited females annually per adult female.
The idea that all those IBWOs out there are quite happily breeding and producing 2+ chicks per year is laughable...
An extremely narrow representation of a paper that reports conclusions in ranges, as it should. In addition you almost certainly took the wrong end of the range for starting N in 1940......perhaps the authors were a bit low also for the IBWO situation in light of the recent news from MS (Jackson, Bonta). The authors of course were looking at an array of variables, conditions and species.....no idea what Illya was attempting.
Many modern researchers strongly believe, with reason, that the starting N in 1940 was > 30. This hypothesis certainly must be considered after the recent report. This being the strong assertion, with references, that there were 12 birds in Mississippi, contemporaneously with the Singer birds, all completely unknown to Tanner. This leads to a very high probability that he missed other birds in addition to these 12 in the '42 population assessment. These birds were in a late seral forest of only 5 X 5 miles. A few patches this size but many smaller existed and each provided potential habitat from 1935 onwards, until some, but not all were lumbered.
His score of months in the field was insufficient to survey the SE US...a huge area with poor roads and by definition wary informants that did not divulge locations easily. His gen was limited.
There is consideration that numbers continued to drop post 1940 following the destruction of the last known habitat blocks and the destruction of other patches that unknowingly, to at least the establishment, had birds. The 40s must have presented various problems for the survivors but the war followed by prosperity took some of the hunting pressure off the woods. The post 1940 population no doubt had foraging problems combined with dismal fledgling rates.
It's highly probable that the numbers could have decreased in the 1940s with either stasis or more losses even into the 60s since the demes at Singer, Mississippi and elsewhere were devastated and new ranges, favorable dispersal pathways or mate selection mechanisms had to be established in fragmented and subpar habitat. Adult survivorship was probably dropping and low fledging rate was not replacing aging birds.
There is no reason to believe that population delta in the post forest destruction period would have been unidirectional and/or linear.
This may explain why sightings seems to have peaked into the 50s and gotten successively scarcer from 60 onwards until 99.
The minimum N may have occurred well after 1940. This starts the Allee clock later.....with obvious viability implications favorable to the IBWO.
It also may partially explain why there appears to be, and may be very few birds today. Since detection rates, even where birds are known to be, is very low the number of birds in relatively unsearched areas is still unclear. Indications are that the species does not densely occupy even alleged optimal habitat patches.
Management or the lack of it may be the key as even good patches may not have the seclusion and density of high energy food items and the increased nest predation implications that comes with excessive foraging time away from vulnerable nests.
tks Fred Virrazzi
Abstract below....
>>>>Increasing initial population size
from 5–30 resulted in extinction rates < 0.05 under limited conditions: (1) all input values were intermediate,
or (2) Allee effect present and annual adult survival ³ 0.8. Based on our model, these species can persist
as rare, as few as five females, and thus difficult-to-detect, populations provided they maintain ³ 1.1
recruited females annually per adult female and an annual adult survival rate ³ 0.8. Athough a demographicbased
population viability analysis (PVA) is useful to predict how extinction rate changes across
