(finally...) A gull with two mirrors, Wollongong pelagic, 25/06/2011.

To: Tony Keene <>, "" <>
Subject: (finally...) A gull with two mirrors, Wollongong pelagic, 25/06/2011.
From: David James <>
Date: Mon, 4 Jul 2011 03:29:08 -0700 (PDT)
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, 


From: Tony Keene <>
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.
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!


Photos, paintings and drawings of Australian, NZ, Swiss and British Birds

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