Cormorant id (Not that easy!)

Not wishing to dispute anything that has already been said regarding this subject on the blog but to try and clarify a number of points I have copied the following article by Julian Hughes which I feel is probably as good as it gets when trying to id which Cormorant species you are looking at in the local area:

Reliable characteristics

Accurately differentiating the subspecies is extremely difficult, and there is some confusion about reliable characteristics. Several of the criteria that we have employed in recent years are no longer as dependable as they were.


Size

P.c.sinensis is, on average, smaller than P.c.carbo, but there is much overlap and significant differences between male and female in each subspecies. In the field, dimensions are usually irrelevant.


Plumage colour

Some older field guides suggest that the plumage of P.c.sinensis has a green gloss and P.c.carbo is blue/purple, but this has now been largely discounted. The colour appears to be dependent on the stage of the reproductive cycle: the green on P.c.sinensis becomes bluish once birds start to incubate.


White head

White feathering (filoplumes) on the head has been suggested as an identification criteria, but this too is unreliable. Both subspecies acquire white heads prior to breeding, with some evidence that older birds have whiter heads. As an illustration, in a colony in coastal France comprised entirely of P.c.carbo, 92% of early breeders had white head feathers, while none of the later breeders showed this feature. The white head fades from egg-laying onwards, so while P.c.sinensis may average more white filoplumes at its peak than P.c.carbo, this is not a reliable character since it is dependent on the stage of breeding. Perhaps one reason why it has been tempting to record white-headed birds at inland sites as P.c.sinensis is because only a proportion of birds in any breeding colony show the characteristic. However, we now know that in the UK, inland colonies have a longer breeding season than coastal colonies, with the first eggs laid in February. Thus, breeding inland is much less synchronous than on the coast, so proportionally fewer birds have white heads at any point in the season. Inland, birds that start the breeding season with white feathering have either lost it or left the area by early April.


Gular pouch

The yellow-orange area of skin at the base of the lower mandible (the gular pouch) dissipates heat from the cormorant’s body. More than a decade ago, Per Alström suggested that the gular pouch is a different shape on the two subspecies, but only recently has this has been critically assessed. Using skins of known subspecies, Stuart Newson has shown that the angle of the gular pouch is a reliable character for assigning the majority of birds to subspecies.






Measured as per the illustration, reproduced from Newson (2000), Great Cormorants with a gular pouch angle of less than 65 degrees are P.c.carbo (A), while those greater than 76 degrees are P.c.sinensis (B). Individuals with a gular pouch angle of 66-75 degrees (which account for around 10% of birds studied in the hand) cannot be reliably identified, perhaps because they are hybrids. It appears that the gular pouch angle is smallest (c.45 degrees) in P.c.carbo birds at the northern edge of the range, in Greenland, and greatest (c.110 degrees) in P.c.sinensis at the eastern end of the range, in China. British birders, at the southern end of P.c.carbo and the western end of P.c.sinensis, thus have a unique identification challenge!

Field identification
Of course, field conditions are very different to measuring the gular pouch in the laboratory (on dead birds!), so even observers with considerable experience of this feature find it difficult to determine the subspecies of every individual. Great Cormorants spend a great deal of time perched, so there are plenty of opportunities to assess the gular pouch, but it usually takes a while before an individual bird holds its head at 90 degrees to its body and in good profile to the observer. Having spent many hours watching Great Cormorants through a telescope inland in the UK, where both subspecies are known to occur regularly, I would suggest that, unless supported by colour-ring data, records of P.c.sinensis in Britain are unreliable on the basis of field sighting alone unless the gular pouch is at the ‘extreme’ end of the range (i.e. 90 degrees or greater).



In my opinion even the photo I took a few days ago just shows how difficult it is to state with any certainty which species you are observing in the field, I would say there is a good argument for two P.c.sinensis amongst the four shown, one definite P. c.carbo and one that I am unsure of.

I think the likelihood is that P.c.sinensis are much more common around here than most people realise.




Finally another adult Cormorant (not the same bird as before) in fantastic breeding plumage and just for the record an unbelievable count of 19 were present on 31 January!