Hi Tony,
It’s great to see someone taking a second look at Silver Gulls. I must confess
to neglecting them for many years. I haven’t seen any comments on your
“two-mirrored” gull so far, it’s not a well studied subject. Sorry, but I
reckon it is an Australian Silver Gull (C. novaeholandiae) and not an NZ
Red-billed Gull (C. novaeholandiae scopulinus).
Firstly, some comments on taxonomy. Sibley & Monroe (1990) were the first to
split them as separate species. Their justification for it was a pers. comm.
from Dick Schodde, without any reasons. It has to be said that gulls are not
one of Dick’s specialties, and he has not published any taxonomy on the group,
either before or in the intervening 2 decades. Splitting doesn’t get much lamer
than that, with no science that I am aware of to support a split. Neither
Johnstone nor HANZAB found any reason to split them. That some field guide
authors follow such splits does nothing to strengthen the lack of arguments in
support. The reality is that they are not very different.
The identification of Silver and Red-billed Gulls is a tough issue. As far as I
am aware it is entirely theoretical, as it has never really been done (in field
or museum) except on geography.
Johnstone recognised that scopulinus usually has 2 mirrors and novaehollandiae
usually has 3. However, the main theme of Johnstone’s paper was that in
Australia there is a huge amount of variation within and between populations.
Regrettably, the HANZAB plumages text for Silver Gull was not as thorough as it
was for some other species in the same volume. It did not have enough detail on
the differences between scopulinus and novaehollandiae (my fault, but that was
all I could do at the time). However, it did acknowledge the marked variation
between ages, sexes, geographical populations and individuals. These different
types of variation frequently combine and interact to mask specific types of
variation. It does note that some individual adult Silver Gulls in
south-eastern Australia have only two mirrors. I have a photo of an adult
Silver Gull from Hobart with two mirrors and just a tiny spot on p7; it is very
similar to this subject bird. I doubt that
the presence of only two mirrors can be used even to rule out an origin from
the nearby Five Islands.
The bill does not look particularly short or stubby or heavy. There is a subtle
average difference between the two forms, but that does not mean that an
individual can be identified by this character. Males have heavier but longer
bills than females, adults larger than juveniles, big-billed individuals bigger
than small-billed individuals, etc. Someone suggested that scopulinus has
brighter more crimson coloured bareparts than novaehollandiae. I would ask, is
that males, v males and females v females, alternate v alternate and basic v
basic plumages across the two forms? Subtle average differences do not really
help to identify individuals in highly varied groups.
So we come to about the only character that might be concrete – the shape of
the mirrors. Admittedly there has not been much study so you can bet that there
is a lot more variation than is realised. However, Johnstone and HANZAB
indicated that on scopulinus the mirrors have a rather square-cut or
perpendicular demarcation at the basal part edge of the mirror, whereas in
novaehollandiae this demarcation tends to be tapered or angular. This bird
appears to have angular demarcation. Given our lack of knowledge, without
square-cut variation, there is not much of a case to prove it isn’t a NSW
novaehollandiae with a dark p7.
Hope this helps. Hope someone has more to add, one way or the other.
David James,
Sydney
==============================
From: Tony Keene <>
To:
Sent: Saturday, 2 July 2011 12:09 PM
Subject: [Birding-Aus] (finally...) A gull with two mirrors, Wollongong
pelagic, 25/06/2011.
Hi all,
Further to the previous emails, here's the photos of the chroicocephalus gull
with only two mirrors from the Wollongong pelagic trip on 25/06/2011. I'd been
very busy with a job interview in Adelaide (my apologies to anyone who I didn't
manage to reply to - many thanks to everyone who gave me information on birding
there!) so I've only just got round to writing this up. I've not included all
the thoughts I've had on this as I didn't want to sway anyone.
http://www.tonykeenebirds.co.uk/random/gull_wollongong.html
Three of the photos can be enlarged by clicking on them and there's a schematic
of the primaries on the port wing.
Any suggestions, ideas, IDs, general frowning, etc, feel free to let me know!
Cheers,
Tony
Photos, paintings and drawings of Australian, NZ, Swiss and British Birds
www.tonykeenebirds.co.uk
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