chrisbrooks's picture

Great Cormorant

Observed: 17th December 2012 By: chrisbrooks
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Great Cormorant

These birds were feeding well out into the old oyster bed lagoons, I think the angle is all wrong to try and gauge the gular pouch angle for subsp. ID, it's difficult enough with the head square on to the camera.

Species interactions

No interactions present.


Bill Henderson's picture

.Gular Pouch Angle


Other reasons for not measuring its gular pouch angle.

1. The bird is still living
2. It hasn't been skinned
3. Its a photograph
4. We don't know what hybridisation does to gular pouch angle.


Bill Henderson

chrisbrooks's picture

Gular pouch angle

Bill, I couldn't have put it better myself, Chris.

As an aside when I last tried to measure the gular pouch angle I had images of both sides of the face of the same bird. The angle differed by about 4 degrees from side to side but it could also have been my dodgy protractor techniques.

ophrys's picture


Well said, Bill! ;)


My Flickr photos...

Ray Turner's picture

Interesting Debate

I find this gular pouch debate interesting in so much as it was on iSpot I learned about it and it was all the rage to use it a year or so ago; the more so because I find myself defending something I’m not necessarily convinced about either.

I do take on board the concerns Bill raises however I believe the technique may be useful at times. Outside of the median values the angles are so different the status of the bird, live or dead, seems unlikely to produce convergence hence an assumption of ssp seems reasonable. If there is a change from an outlier it would be to bring it to the mean and therefore no identification to sub species would be made anyway so no false identification.

The same argument is also true regarding the use of photographs given the picture is of sufficient resolution to allow reasonable measurement and the bird is approximately level with the observer. Rotation about the y axis will not affect apparent angle (though it will make it more difficult to measure) but rotation around the x axis will reduce the apparent angle. So given the aforementioned caveats making a judgment from a photo need not be an issue. The same is true of measurement error; four degrees in this context is neither here nor there.

The most valid argument then is the affect of hybridisation. Anyone know of any work done on this?



Bill Henderson's picture

Re Interesting Debate


You raise a number of points which trouble me.. I'll try to cover them in the order presented. I am assuming that we are both basing our arguments on the reference you were kind enough to supply earlier:

Para 2

Median, there is no mention of median. The paper refers to mean +/- SE. SE or standard error can be thought of as a measure of confidence in the mean. There is an overlap in the range of measurements made: 38 to 72 for carbo and 66 to 111 for sinensis.

Para 3

Rotation about the vertical axis DOES give rise to a change in the apparent angle.

4 degrees is a sizeable difference between both sides of the same bird.

Para 4

All of what I said was valid and not just the effect of hybridisation.


Bill Henderson