The sand banks off East Anglia host an annual wildlife spectacle which goes almost un-noticed, as David Thompson of the Sea Mammal Research Unit explains.

Between early June and mid July, hundreds of female common seals (Phoca vitulina) haulout to give birth to and suckle their pups, while males perform some of Nature's most dramatic mating displays. Pups weigh around 10-12 kg at birth and are usually born with a sleek, grey-black coat. Occasionally one is seen with a fluffy white coat similar to the more familiar grey and harp seal pups.

In fact all pups develop this white coat, but they usually moult in the womb. This is an adaptation to their early lifestyle. While grey and harp seal pups lie on beaches or float around on ice, common seal pups are born on tidal sand banks and must swim within hours of birth. Fluffy white fur provides insulation only when dry, for a swimming seal it would just add drag.

Pups grow rapidly on a diet of fat-rich milk, more than doubling in size in three weeks. Their mothers lose a third of their weight, using two-thirds of their energy stores and so continue to feed during lactation, making regular forays which get longer as the season progresses. Some females take their pups with them, but others leave them at haulout sites. It is these 'abandoned' pups that are often mistakenly rescued and taken to rehabilitation centres. At the end of lactation the females come into oestrus and leave their pups. By this stage, healthy pups have more than doubled their birth weight and have laid down large reserves of blubber which they will live off during the first few weeks of independence while they learn to hunt.

Males have been more or less absent during this time, spending June feeding or resting on haulout sites away from the females. As the mating season approaches males become more aggressive and there are frequent fights. Then in early July they begin to sing. From chosen sites they perform long sequences of five minute dives, calling underwater once a minute. It is not a particularly melodious song, and sounds like a bowling ball rolling down an alley and hitting the pins! Presumably the females find it attractive. On the surface, the only signs of all this activity are occasional fights and loud gunshot noises produced by males whacking the water surface with their flippers.

The bulk of England's common seal population is found in the Wash where they haul out on offshore sandbanks and along tidal creeks. There are also large groups at Blakeney Point and Donna Nook, in Lincolnshire, and smaller groups on Scroby Sands, off Yarmouth and in the Thames, Blackwater and Crouch estuaries. The size of this population has varied dramatically. Throughout the 1960s and early 70s there was an intensive pup hunt, with a large proportion being killed each year.

Once hunting stopped numbers increased to around 4,000 in 1988, only to be devastated again that autumn when around half the population died of phocine distemper. The epidemic spread through Europe during the breeding and moulting seasons, killing over 16,000 seals. It was three years before the English population showed signs of recovery, but fortunately counts have now increased to around 3,500.

Substantial numbers of grey seals also haulout north east of the Wash at Blakeney Point and Scroby Sands where several hundred may be seen during spring and summer. Grey seals do not breed in East Anglia, however there is now a large and growing breeding colony to the north of the Wash, at Donna Nook. This grew rapidly during the 1980s and 90s, and is the only mainland site in Britain with direct access for the public to observe breeding grey seals.