Here is a picture of a cormorant on the Serpentine, sent by Johanna van de Woestijne, who commented on this blog earlier. She took it on 3 March last year. She asks whether it is the British species Phalacrocorax carbo carbo or the European mainland species rather confusingly called P.c. sinensis.
It is said that sinensis can be identified by its having white streaks on its head, like the bird in the picture. However, both subspecies grow these white feathers as the breeding season approaches.
The only reliable indication is the shape of the gular pouch, the patch of orange or partly orange skin at the base of the lower mandible of the bill. If the angle of the upper rear corner is acute, it's the ordinary British species. If It's obtuse, it's the European _sinensis_ subspecies. The difference is illustrated on this page.
The bird in the picture has an acute angle, so it is a normal British cormorant, P. c. carbo. But sinensis birds have been seen on the lake, by Des McKenzie.
The number of cormorants in the park has fallen sharply this year. Previously they used to turn up mob-handed and fish the lake in line abreast in an alarmingly organised fashion. We think that they have eaten all the fish that were of a suitable size, and that is why one seldom sees more than two or three now.