Saturday 17 March 2012: Cormorant rookery
Another grey spring day, though at least we had a little rain first thing. I spent much of the morning key-wording my photographs, with a brief trip to PC World to get a replacement hard drive for Chris's laptop,as his decided to fail in the middle of writing his dissertation. Luckily he's conscientious about backing up, and has hardly lost any data - phew!!
After lunch the sun made a feeble attempt to break through the cloud and Chris and I headed off for a walk round Holme Fen. Everything was fairly quiet untl we got to the mere. I last blipped this on 2nd February, when the cormorants were beginning to pair up. We could hear a cacophony of bird vocalisations as we approached the water's edge, and eventually realised that this was the incessant call of cormorant chicks, interspersed by the deep braying of the adults.
We managed to find a spot with a relatively clear view of the rookery, which had a significant number of nests. Several of these contained very large chicks - the one shown above has four, all of which look well fed. There was at least one other nest with four slightly smaller chicks, and quite a few birds still seemed to be sitting on eggs.
At one time cormorants were considered to be almost exclusively coastal breeders, and many still nest on cliffs and rocky outcrops. In 1981 they started establishing inland colonies, usually nesting in trees, as at Holme Fen, where they are using birches. In the quarter-century to 2005, breeding had been recorded at 58 inland sites, and the inland population rose to at least 2,096 pairs, exceeding the coastal total of 1,564 pairs (Mitchell et al 2004, Newson et al. 2007).
The cormorant is a cosmopolitan species, breeding in every continent except South America and Antarctica, with probably six races/ subspecies. The nominate race carbo nests on rocky cliffs from eastern North America, through Greenland and Iceland to the British Isles and Norway. The sinensis race is found from China (as its name implies) through India and across continental Europe where they usually nest in trees.
Detailed study (Newson et al. 2007) at colonies in eastern and central England, including observations of colour-ringed birds and DNA sampling, suggests that inland breeding has probably been sparked by birds of the continental race sinensis from the Netherlands and Denmark, but many carbo chicks from coastal colonies in Wales and England have also moved to inland sites to breed.
The proportion of carbo increases in longer-established colonies, suggesting that inland colonies might be founded by sinenis but more and more carbo then join them. Cormorants are faithful to their natal colony, but as a site nears its carrying capacity an increasing proportion of mostly younger birds breeds elsewhere, either by moving to existing colonies or founding new ones.