Friday, February 18, 2011

New ID tool?

Recently I have read a few different discussions about white wingers and whether or not they were Iceland or Glaucous. Now although most can be assigned to species level pretty easily, there are always a few that verge on large (male) Iceland and small (female) Glaucous. So after having a quick think I managed to make a link between keeping corn snakes and Bird Identification! When I used to keep quite a few corn Snakes there was a programme that you could download for free that would tell you how long your corn snake was. A quick internet search and I had found said programme and quickly downloaded it.

So what is this programme? Well you can download it here and it works for snakes by taking a photo of a snake next to a known length ie a ruler. Then you measure the length of the snake and it uses ratios to tell you how long your pet is.

Now this got me thinking, Glaucous Gulls on the whole have a deeper bill than Iceland Gulls (don't have BWP/i or any ringing books so can anyone confirm this for me?) and so could I use this programme to get some kind of comparison between two different photos?

Now the first flaw to this is that it would be kinda tricky to take a photo of these gulls conveniently posed next to a ruler! However I thought maybe I could use the eye to get a scale of eye to bill depth.

So heres how I did it, I got some photos of the internet of various Iceland and Glaucous Gulls and used this programme to obtain a ratio for eye size to bill depth. Eye length was measured from the bottom to the top of the actually eye in a vertical line and bill depth was measured from the tip of the gonys angle vertically up (so on some I had to compensate depending on the angle the bird was holding its bill at)

So the results from a quick experiment involving 3 Glaucous Gulls and 3 Iceland Gulls from google images.

Ratios as Eye:Bill Depth

Glaucous Gull 1= 1:1.911
Glaucous Gull 2= 1:2.408
Glaucous Gull 3= 1:2.058

Iceland Gull 1= 1:1.991
Iceland Gull 2= 1:1.701
Iceland Gull 3= 1:1.756

So even in this tiny study size you can see that Glaucous Gull ratios range from 1.9-2.4 and Iceland 1.7-1.9.

Of course it would be a lot more accurate to do more birds and maybe I'll get round to it at some point.

Now the whole inspiration for me to do this was a post by Alan Tilmouth (nothing to do with me having a reading week at uni!) of a bird he'd seen and wasn't sure if it was Glaucous or Iceland. 

So I thought I would use the programme on a photo from this post and got a ratio of 1:1.608 so from my small study above falls into the Iceland Gull category.

Also found this thread on surfbirds

Ian Lewington

Again even though this bird was settled as a Glaucous Gull I did the ratio on the bird above and it came out as 1:2.423 so in the Glaucous category.

Be interesting to hear of some feedback of what people think to this.

Could this be a useful tool for IDs of birds like this and similar?
Anyone else wanna have a play with the programme and see if you get similar results as me?
Any other species/taxa groups this could be useful for?


  1. Tim,
    I did a bit of work looking at bill depth/width ratios in Storm Petrels. I was using live birds in the hand and a set of callipers; I also measured eye-width, hoping it would give me some sort of stable measure against which everything else could be compared. It didn't. It was extremely variable and often held no relation to the size of the bird or the bird's bill.
    Not sure eye 'height' is the best measurement to be using, either. What happens if you get a sleepy gull or a photo taken half way through blinking?! Be very careful with foreshortening as well if the image isn't taken perpendicular to the subject.

  2. Cheers for that input Stephen, have since done some more ratios using this programme and I'm finding that its very variable even with just how I measure it. The photos I've picked I've tried to pick ones that the bird's eye and bill are in a line so as not to change the size perspective too much and also birds that appear to have their eyes fully open.

    Some birds however seem to have small eyes and large bills or the other way round which also makes things a lot more confusing!

  3. Tim check my bird? or maybe you need a better photo?

  4. An really interesting post Tim - I'll try it out on some pix of birds here in Shetland where we are lucky to get to see lots of Glaucs and Icelands.