Dion's comment on Patagonia makes me wistful about that spot in south-east
Arizona that I visited but briefly a few months ago.
The Picnic Table Effect that he mentioned is in fact a salutary example of
records that lack independence. It was coined for Sierra Madrean birds that
were apparently straying north from Mexico, but were actually associated
with a highland habitat that was relatively uniform and extending across the
border.
The Australian Shelduck is definitely rare in the Northern Rivers. Dennis
Gosper and I have spent over 50 birding-years there and obtained only one
record. Our observed frequency is therefore about the same as for Long-toed
Stint, American Golden Plover, Green Pygmy-goose, Great Crested Grebe and
Australian Pratincole in the wetlands of this region. By contrast, we found
birds such as the Ruff and Pectoral Sandpiper to be much more numerous.
Suppose we become quite sanguine about the prospects of observing the rare
duck trio in the Grafton wetlands and try to calculate the probability of
all occurring in the one year. Let's assign actual [not observed]
frequencies of occurrence with generous values as follows...
Northern Pintail -- 1 year in 20
Northern Shoveler -- 1 in 10
Australian Shelduck -- 1 in 5
The probability of all in the one year becomes 1/20 x 1/10 x 1/5 = 1: 1000.
That's one year per millenium !! The probability of coincidence in one
month [August 2000] would be considerably less than that figure.
Clearly these records are unlikely to be independent. I suggested trying to
determine the possibility of escape simply as the most tangible path of
investigation. After all, several birders were compelled to untick a Ruddy
Shelduck near Toowoomba not too many years ago when it was proved that it
had escaped. Some have suggested that it is sufficient to examine the
registry of captive birds, but in my experience there are many unregistered
birds in existence.
If captive birds are not the source, then environmental events should be
analysed. These would be of the type best witnessed among seabirds, where
weather or extreme water temperatures cause the coincident occurrence of
rarities.
Perhaps the onset of drying in an abnormally wet arid zone has begun to
concentrate waterfowl in coastal refugia such as the Clarence. Time may
tell.
Glenn
Glenn Holmes & Associates
*birding & botanical specialists*
PO Box 1246 Atherton Qld 4883
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