I photographed this Cormorant sat in sunshine the other day.
We now know the straight drop at the proximal end of the gular pouch as shown here is indicative of ‘sinensis’; Continental cormorant.
‘carbo’; the British race has a smaller pouch and the lower edge is more aligned with the bill, not at right angles to it. The continental cormorant has therefore a larger gular pouch.
Why has ‘sinensis’ got a larger gular pouch?
One of the uses of the pouch is to loose heat. As the bird pants it uses the airflow over the thin skin and shallow blood vessels to radiate heat. It’s supposition on my part, but perhaps because ‘sinensis’ is from a warmer climate and needs to loose heat more readily … the pouch is larger.
We don’t see ‘carbo’ often in Norfolk now. It turns up more frequently when we visit Wales or Scotland on tour. The continentals have pushed out the British race to the west. Why? Because ‘sinensis’ are from a warmer climate where the breeding season is longer perhaps their natural cycle means they breed earlier in the year than ‘carbo’? First nesters get prime sites. Maybe British Cormorants get pushed out from the best nesting sites so they become less successful breeders. Also perhaps because ‘sinensis’ are from a warmer climate where the norm is a longer breeding season they have time to raise two broods rather than one so they out populate the slower breeding British race? I don’t know – more research required?
What caused the ingress of Continental Cormorants to our shores in the first place? Global warming would seem to be an appropriate fit, wouldn’t it?
Letter From Norfolk 
I see a lot of cormorants down here on the Catalan coast where I love, including some brown ones. There are still a lot of wetlands and wildlife parks where they can thrive (e.g. on the Delta de L’Ebre). George Monbiot argues that British nature reserves are glorified farmland – could that also be a factor in the decline of the British cormorant?
Thank you for your comment Alistair. You do indeed live in a beautiful part of the world.
George Monbiot may be right, who knows, but I don’t think this would be the reason why British Cormorants are moving further west and north. If it was purely down to Nature Reserve habitat quality and availability the Continental Cormorants wouldn’t be here either!