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Frank,
Ross posted links to images that appear to
show a typical silver gull and a typical red-billed gull (still both Larus
novaehollandiae). They illustrate the presence or absence of a mirror on P8
nicely.
What I wrote was:
"There seems to be some OVERLAP but the differences
can be summarised as GENERALLY..." (I've added the caps)
The reason I wrote this is no one character appears
to be definitive (inc the presence or absence of a mirror in P8). For
example I have copies of photographs taken in NZ by Brent Stephenson that
show a putative adult Red-billed Gull (identified on range) with a very small
white mirror on P8. What I would argue is that in combination the presence or
absence of all characters I listed (bill structure, tone of grey on dorsum,
mirror in P8 etc) would identify an individual as either a red-billed or a
silver gull.
Hope that helps
Rohan
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