Broadcast 23 December 1998, the final programme investigates the challenges that must be surmounted if birds are to survive. The sandgrouse is a species that has adapted to desert living: its breast feathers are capable of absorbing water, which it can pass on to its young. The crab plover also nests in the sand, and burrows until it finds a comfortable temperature. Birds that choose remote places can proliferate hugely, like the flamingos on an African soda lake. Meanwhile, during winter, the entire world population of spectacled eiders can be found in just a few assemblies on patches of the Arctic Ocean. The city is a relatively recent habitat, but many have become accustomed to it, such as the American black vultures in Sao Paulo. In Japan, crows have learned to crack nuts by dropping them on to pedestrian crossings - and waiting for the traffic to stop before collecting them. In North America, purple martins have become totally dependent on humans for their nest sites. Attenborough highlights man's influence by describing the Pacific island of Guam, whose bird population was wiped out following the accidental introduction of brown tree snakes during the 1940s. Examples of species that were hunted to extinction are the huia, the great auk and, most famously, the dodo. However, there are conservation efforts being made, such as those for Australia's orange-bellied parrot, the pink pigeon and the echo parakeet (the latter two both of Mauritius).