The Mute Swan is a very large white waterbird. It has a long S-shaped neck, and an orange bill with black at the base of it. Flies with its neck extended and regular slow wingbeats. The population in the UK has increased recently, perhaps due to better protection of this species. The problem of lead poisoning on lowland rivers has also largely been solved by a ban on the sale of lead fishing weights. Some birds stay in their territories all year, while others move short distances and form winter flocks. In cold weather, some birds arrive from Europe into eastern England. The Mute Swan is one of the heaviest flying birds, with males (known as cobs) averaging about 12 kilograms (26 lb) and the slightly smaller females (known as pens) weighing about 9 kilograms (20 lb).