Broadcast 18 November 1998, the next programme details river and ocean dwellers. The dipper swims completely below water to search for food, whereas the kingfisher uses a 'harpoon' technique, diving from a vantage point. However, the darter uses a combination of both methods, stalking its prey underwater before spearing it. By contrast, the reddish egret uses a kind of dance to flush out the aquatic inhabitants. Skimmers have different-sized mandibles, the lower one being used to skim the water's surface for small fish. Ducks have developed an assortment of angling skills. Some dabble, like the mallard, while others are of a more streamlined design and are at home underwater, such as the merganser. Waders, which specialise in feeding on mud flats at low tide, include avocets, godwits, dowitchers and sanderlings. The pelican feeds in groups, their pouch-like bills being more successful when used collectively. Boobies fish in the open ocean and are shown dive-bombing shoals en masse. Attenborough visits Lord Howe Island, off Australia, and by imitating the calls of various birds, invites a group of curious Providence petrels â€” which are indigenous â€” to investigate. Because there are no humans in their habitat, they are a very trusting species, as Attenborough discovers when one perches on his hand. Out on a seemingly empty area of ocean, the presenter is able to fill it with various sea birds within seconds, simply by throwing fish oil on to the water.