Friday 21 November 2014

Redwings: coburni v iliacus

I think I was a little too cautious in my post 'More of the same' (link here) with regard to the photograph of a Redwing that showed characters of the Icelandic race 'coburni'. I suggested its appearance possibly fell a little short of the mark but if anything fell short it was my experience of birds from the Icelandic population (nil up to then). I caught another dark and heavily marked bird yesterday and it stood out as much if not a little more than the previous bird did which prompted me to reconsider that earlier note of caution. It also prompted me to do some homework and I have been trawling the internet looking at photos of the races and reading up on the subject. I am now certain that yesterday's bird and the previous bird (caught 16/11/14) are Icelandic Redwings.

Birds of the Icelandic subspecies (T.i.coburni) are marginally larger and darker than nominate birds from northern Europe (T.i.iliacus) but only around 14% of Icelandic birds have longer wing lengths and therefore relatively few can be separated on size. Both birds had wing lengths that fell in the upper part of the overlap in the wing length ranges with yesterday's bird being 127mm (right on the upper limit) and the first bird had a wing length of 124mm. Published wing length ranges vary depending where you look but I have gone with 126/127mm as being the upper limit for nominate birds largely based on a paper that considered occurrences on Heligoland (Germany) which can be found here.

In terms of appearance 'coburni' is described as being slightly darker and are generally more heavily marked and having looked at a lot of images of both races there appears to be less overlap in appearance than there is in size. However I do feel there was a bias towards showing obviously darker birds amongst the examples of migrant 'coburni' photographed outside Iceland which is only to be expected when the separation of the races is not always clear cut. Another feature for separating the races is the colour of the legs and feet which are dark horn brown in 'coburni' and pinkish-flesh in 'iliacus' and this was very obvious and can been seen in many of my photographs.

Redwing of the Icelandic race 'coburni', Billinge 20/11/14
The following series of photos shows the same bird with a fairly typical bird of the nominate northern European race 'iliacus' that was caught the same day and photographed in similar conditions. 

'coburni' above and 'iliacus' below.
'coburni' left and 'iliacus' right.
'coburni' above and 'iliacus' below.

The following images are of the previous Icelandic bird caught on 16/11/14. On looking at these images again I don't know why I was cautious about fully attributing it to the Icelandic race. 

Redwing of the Icelandic race 'coburni', Billinge 16/11/14

Redwing of the Icelandic race 'coburni', Billinge 16/11/14 (same bird but appears to have a slightly paler throat and upper-breast from this angle).

Redwing of the Icelandic race 'coburni', Billinge 16/11/14

...and here is a collage showing how it compared with a lightly marked nominate bird caught the same day.

'coburni' left and 'iliacus' right.
The final image below shows a collage of both Icelandic birds with what I consider to be a heavily marked nominate bird (lower right) and a more typical nominate birds (top right). Although you can't really see much of the upper parts you can see enough to appreciate how much darker both Icelandic birds are. The upperparts of the Icelandic birds are what I would describe as a dark olive earth-brown whereas the nominate birds are more of a mid toned warm brown. The other thing to note in this image are the feet; darker horn brown coloured in the Icelandic birds (left) and pinkish-flesh coloured in the nominate birds (right). The strength, size and colour of the markings on the underparts largely speak for themselves but there is a little more variation in them.

Upper left 'coburni'; lower left 'coburni'; upper right 'iliacus'; lower right 'iliacus'.

If you are interested in comparing other images of migrant 'coburni' photographed in the UK examples can be found  here (Bardsey),  here (North Ronaldsay),  here (Fair isle),  here (Norfolk).

For images of 'coburni' taken in Iceland you can view images taken by Jakob Sigurdsson here and here. Other images by Gudmunder Geir's can be found here.

It is also worth a second look at the 'We Bird North Wales' blog post on this subject here.

No comments:

Post a Comment