The Bittern [above] and Great Bustard [below] both became extinct in the UK in the 19th century due to hunting and habitat loss. There are now very small numbers of both birds in specific locations.
While the paintings improve the surroundings of housing estates and give people something to appreciate on city walls, the main idea is to remind people about birds, their beauty and significance and what conditions they need to flourish; hopefully to inspire people to do more to help them and nature generally.
For example: don't cement over a front garden to make way for a car -- instead keep a privet hedge, or plant a new one, where House Sparrows can find shelter. It often doesn't take too much effort to dig a small pond in a back garden or, on a community level, convert a patch of industrial wasteland into a garden. But this can have a huge impact.