Cormorants have been the subject of considerable interest at Paxton Pits since the early 1980s, when numbers started to increase, the species benefiting from the increased availability of food at Grafham Water reservoir and former gravel pits in the Ouse Valley.

For birdwatchers, they have been a source of fascination – the return to the fenland basin of a species which had been rendered extinct as an inland breeding species with the arrival of Vermuyden’s drainage team in the seventeenth century. For anglers, the return of these birds has been viewed with greater suspicion – while the cormorants have been away, leisure angling has developed and with it the practice of stocking lakes and reservoirs with fish. Grafham Water and Paxton Pits have brought these conflicts into sharp relief.
The historical perspective
Since the recording of birds began, cormorants have been rare in Cambridgeshire. Indeed, there were only two records in Huntingdonshire during the nineteenth century, with a further six recorded in the county by 1923. The cormorant’s distribution changed little between 1875 and 1980, with most inland breeding colonies already extirpated from the UK by 1800, though a colony of 50-60 pairs lasted near Herringfleet in Suffolk until 1825. There are, however, records of cormorant bones from the Cambridgeshire Fens, presumably from the days before the Washes were created and the marshes drained.

As gravel was extracted on a major scale in the Ouse Valley from the 1930s, so a new source of food became available for wintering cormorants. The creation of Grafham Water in the 1960s accelerated the trend, with maximum counts increasing from three in 1966, 45 in 1983, 240 in 1987 to 670-675 in 1992/94. The increase was reflected at several other sites in England.

Breeding cormorants at Paxton Pits
Although cormorants had been recorded at Paxton during winters in the 1980s, it was 1988 before a pair bred at the Pits, when a pair established a nest in the heronry. They possibly laid eggs, but this is not confirmed because the nest disappeared soon afterwards. In 1989, three nests were built, from which the first young were raised in Huntingdonshire for several hundred years.

The breeding population increased rapidly, as shown in the graph below. Numbers peaked in 1996, at 218 nests, but then fell to 90-130 nests each year until 2005, when the count increased again. The reasons for this are unknown, though various suggestions have been put forward, including:

• changing availability of food (partly through changes in fish stocking regimes at Grafham Water in the late 1990s in response to predation by cormorants)
• availability of nesting sites as older trees within the colony have died
• anecdotal reports of illegal shooting of cormorants as birds fly between Paxton and Grafham

Breeding population of cormorants at Paxton Pits (nesting pairs).

Wintering cormorants at Paxton Pits
The trend in numbers of breeding birds at Paxton is also reflected in the wintering counts. For example, in winter 1987-88, counts varied between six and 80, but by 1992-93, the peak count was over 250 birds. The all-time peak count at Paxton was 1153 birds in January 1997, but winter counts have fallen since, to a typical peak of 400 birds.

Cormorant movements from Paxton
During the mid 1990s, a major ringing programme was undertaken to better understand the movements of cormorants from inland colonies at Abberton Water (Essex), Besthorpe Gravel-Pits (Nottinghamshire), Rutland Water (Leicestershire) and Paxton Pits. Colour-ringing at Paxton began in 1995, though operational constraints prevented it proceeding beyond 1996, by which time 37 juvenile birds had been ringed.

Of the 37 young cormorants ringed during 1995 and 1996, 43% were resighted (16 birds on 59 separate occasions) by the end of 1998. Birds from Paxton were seen at 20 different sites, including Paxton itself (responsible for 10 of the 59 resightings –a much lower resighting rate than the other four colonies). However, there were also sightings at Paxton of 23 cormorants from Abberton, Besthorpe and Rutland.

All of the birds ringed in the Paxton colony were resighted in England, birds travelling as far as Yorkshire, Shropshire, Suffolk and Berkshire. Young birds from other colonies were seen as far away as Spain and Tunisia, though none of Paxton’s birds were found abroad. Birds were found an average of 90-120 km away from their natal colony, with first-year birds leaving their breeding colonies within two months of fledging and being found farthest away. Adult birds, by contrast, were generally found much closer to the breeding colony.

It would appear from the ringing data that the birds which breed at Paxton Pits and those which winter there are almost entirely different populations. Breeding birds return to the colony from January, which may well explain the fact that ‘wintering’ numbers appear to peak in late January or early February. Indeed, in some years, the first nests are already under construction by the end of January.

There are two races of cormorants in Europe: Phalacrocorax carbo carbo, which breeds in Britain and Ireland, and P.c.sinensis, which has traditionally been considered a continental subspecies. Birds at Paxton are mostly the carbo race, though only two birds ringed elsewhere have been found breeding at Paxton – one from St Margaret’s Island (Pembrokeshire) in 1994 and one from Grune Point (Cumbria) in 1995.

One of the key conclusions of the ringing studies is that the continental subspecies sinensis is commoner at inland colonies in England than had been realised. This may partly be a result of improved identification techniques – it is only in the last few years that accurate identification features have been described. Based on the shape of the gular patch (the orange skin at the base of the bill) and microsatellite DNA studies, birds of the sinensis race breed at Paxton, but field identification is very difficult. For more information about identifying the two races, click here.

The future of the Paxton colony
The colony grew from nothing in eight years, but declined for a decade after 1996 and has since increased once more. Monitoring in Denmark suggests that colonies reach a certain level and then numbers plateau as new satellite colonies develop nearby. However, although wintering birds continue to feed and roost at Grafham Water, Fen Drayton Gravel-Pits, Marsh Lane Gravel-Pits and as far north as the Nene and Ouse Washes, only small numbers breed elsewhere in Cambridgeshire.

It may well be that cormorants benefited unduly from artificially high numbers of fish in stocked lakes and reservoirs during the 1990s, but that changes in stocking practices have now reduced the availability of ‘easy food’. On the other hand, cormorants may be a barometer of the environmental quality of local waterbodies, so a decline might be an indicator of more serious problems.

We can continue to watch the changing fortunes of Paxton’s cormorants with interest.