Ray Turner's picture

Cormorant

Observed: 9th November 2012 By: Ray Turner
S159 Neighbourhood Nature - course complete
Ray Turner’s reputation in BirdsRay Turner’s reputation in BirdsRay Turner’s reputation in BirdsRay Turner’s reputation in Birds
DSC_5482 10
Description:

Lots of cormorants at the LWC yesterday.

Identifications
Species interactions

No interactions present.

Comments

Ray Turner's picture

Identification Note

Could not find a protractor so did it the old school way, with a pair of compasses. Subtended 90 degrees to get 45 degrees, subtend the result.

45 + 22.5 = 57.5 is darn close to 58 degrees I’d say, hence P. c carbo. Who says geometry is dead?

Ray

Ray

chrisbrooks's picture

Maths

67.5 ?

Ray Turner's picture

Opps

That’s why I only have two thirds of a maths degree LOL

Ray

Ray

Ray Turner's picture

Should of course be:

Should of course be: subtended 90 degrees to get 45 degrees, subtend the result, subtend once more.

45 + 11.25 = 56.25 is darn close to 58 degrees I’d say, hence P. c carbo.

Q: Why do you rarely find mathematicians spending time at the beach?
A: Because they have sine and cosine to get a tan!

Ray

Ray

Bill Henderson's picture

Optical Delusion

Hi

Can this method of measuring angles from photographs fo ID be relied on?

Looking from behind would have the effect of increasing the angle. Looking from above or below would decreae the angle.

Does anyone have access to a stuffed bird to try this out?

Bill

Bill Henderson

Ray Turner's picture

Hi Bill

I’m not sure I agree with the premise concerning looking from behind. Rotation about the vertical axis should leave the angle unchanged, though it would be correct to say looking from above or below would change things.

However let as assume the argument were correct then we are saying the angle for this particular bird would be a little less when viewed directly side on.. In which case the sub species for this bird would still be P. c carbo as the range for P. c carbo is 38 to 72 degrees (mean is 59.7 plus or minus 1.11) though most of this range is due either to sexual differences or regional variation. P. c sinensis on the other hand is 66 to 111 degrees (with a mean of 86.2 plus or minus 0.65) which is well outside the range for this bird.

Ray

Ray

Bill Henderson's picture

Angles

Ray
I've just sellotaped (Sp?) two pencils together and looked at them from various directions. Looking from behind or from in front has a definite effect on the perceived angles.

Further, I'm not sure that we should be concerned so much with the averages but rather where the angles are closest or overlap. I don't know where your plus or minus figures come from.

I'm probably holding the spongy end of the stick. Be generous and put it down to beginners' enthusiasm :-)

Bill

Bill Henderson

Ray Turner's picture

Enthusiasm

Nothing wrong with a bit of enthusiasm Bill, beginner or otherwise. I’ll have a go at repeating your experiment when I get a moment and see if I can measure the apparent angles as they are rotated around the axes, could be fun.

As for the figures; have a look at this paper
www.avibirds.com/pdf/a/aalscholver1.pdf

Ray

Ray

Bill Henderson's picture

Figures

Ray

Thanks for the reference. I've only had time to scan it but promise to re-visit it a little later on today.

However, there is one thing that jumps out at me; that the survey was carried out on dead birds. Not just dead birds but the skins of dead birds. My cursory glance didn't find mention that this could be applied to photos of living examples. I think that this is probably a step too far.

This is a great "newsgroup" because it doesn't stamp on beginners like me when we ask naive questions like this.

Bill

Bill Henderson

Bill Henderson's picture

And again

I've had another look at the paper and there is brief mention of live or newly dead specimens "On live or freshly dead specimens, it is important to note that the gular pouch can be distorted to change its angle by pulling outwards and inwards. With all measurements, care was taken to ensure that the pouch assumed is normal attitude by gently stroking it downwards before measurements were taken".

I don't know why they mention live specimens because they don't appear to have used measurements from them ins this study. What does seem to be increasingly clear is that even on dead animals or their skins great care needs to be taken. I am now nearly convinced that this method, as it stands, can not be reliably applied to photographs. More work would have to be done here.

In the discussion were are told that hybridisation could affect the reliability of the results.

"In this study we assume that cormorants belong clearly to one or other of the two sub-species. However, hybridisation between the two sub species has been demonstrated..... although the level of hybridisation occurring and the influence of this is unknown. For this reason, the discriminant functions here may not be reliable if tested on samples that contain hybrids, although...."

Bill

Bill Henderson

Bill Henderson's picture

Mistake

Hi

I posted here by mistake and have removed my posting to where it should be.

Sorry

Bill

Bill Henderson