Tuesday, 27 October 2015

Redwings: coburni v iliacus 2015

I nearly didn't go out this morning because the wind was a little stronger than had been forecast and was near the limit for mist-netting at Billinge. After umming ahhing for bit I decided to give it a go, after all it is late October, the wind had turned south easterly again and you just never know what may turn up. I was reasonably sure there would be an increase in Redwings as there had been a small arrival on the east coast and on the northern isles the previous day including some Icelandic birds on Fair Isle and North Ronaldsay. As a general rule there is a passage of thrushes over Billinge 24 hours after arrivals on the east coast from Spurn northwards and this proved to be the case again this morning.

The first round of the nets produced 13 Redwings and 2 Goldcrest which I would have been happy with as a total for the morning given the conditions. The next round produced another 7 Redwings and 7 Goldcrests which indicated there had been an increase in Goldcrest numbers too. The catching rate then slowed as the cloud cover thinned and the sun tried to break through but there were a few birds in every net round up to 10:30 when I decided to pack up. The final total of 53 new birds exceeded my expectations by some margin and was made up of 28 Redwings, 1 Song Thrush, 21 Goldcrests, 2 Bullfinch and 1 Lesser Redpoll.

Bird of the day and the main reason for this post was a Redwing that I considered to be of the Icelandic race 'coburni'. When I saw it at the far end of 18m net I knew it was likely to be one, assuming I am correct of course. It was darker than any of the other Redwings, was more heavily marked, had striking undertail coverts and even felt bigger when extracted and was in the hand.

Icelandic Redwing (coburni) 27/10/15

Icelandic Redwing (coburni) 27/10/15
Many years ago I read a comment about the identification of Goshawks which said something along the lines of  'when you think you have seen a Goshawk you haven't because when you do see one you will know'. The same seems to apply to the identification of Icelandic Redwings, when you only think you may have caught one you almost certainly haven't but when you do catch one you will have no doubt. As well as looking good for 'coburni' it had a wing length to match and measured 128.5mm. Now I know that is only 0.5mm over the maximum for 'iliacus' and I would never claim one on the basis of that 0.5mm alone but I would have been equally happy with the identification had its wing length fallen well within the overlap between the races.

Now ideally this identification needs to be confirmed in some way and the bird left the odd stray body feather, as thrushes often do, which was collected up and saved for possible confirmation via DNA . Luckily I share this interest in the identification and occurrence of Icelandic Redwings with Martin Garner of Birding Frontiers and with Martin's help and contacts there may be the opportunity to see if the races can be separated via their DNA. If there is any progress with this I will post about it in due course but up until then you can either take my word for it or make your own mind up from the photographs.

RZ06779 is the 'coburni' Redwing and RZ06780 and  RZ06782 were the following 'iliacus' caught and ringed this morning. You may be wondering why I didn't use pictures of RZ06781 and the simple answer is because it was a Song Thrush, it was not that I was being selective in the 'iliacus' used for comparison. Note the leg and feet colour of the 3 birds, 'coburni' is dark chocolate brown while 'iliacus' is mainly pinkish straw although a few can be brownish but certainly not as dark as 'coburni'.

The two birds on the right show significant variation in the extent of the flank streaking of 'iliacus' and show why the differences are not always clear cut and easy to describe.

Same birds and the 'coburni' on the left should be jumping out now if it hasn't done before. If they all had undertail coverts like RZ06779 the job would be easy but unfortunately that may not always be the case. More photographs of other individuals are required.
Please don't get the idea that I think you can always separate the races from their overall appearance or on the basis of specific plumage features as I just don't have enough experience of 'coburni' to say that at this time. It may be possible but at this stage I just don't know for sure and that is why it interests me so much. There may be some overlap in appearance as there is with wing length and the danger with separating very similar races is in focusing on the differences between easily separated individuals whilst understating any similarities that may occur at the other end of the scale.

The final picture (for now) shows today's 'coburni' (left) with a bird that I commented on in a previous post because of its size but was happy with as an 'iliacus', You can see differences but it is far from easy and would be much more difficult at range in the field. I am still happy that the bird on the right is an 'iliacus' despite its size and fairly well marked breast.

Hopefully I will catch and photograph a few more Icelandic Redwings before the autumn is out but if you would like to see my previous post on this subject you can find it here.

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