The common house sparrow (Passer domesticus) is recognised the world over and has been an extremely effective coloniser when introduced to new habitats. In the 1860s they were introduced by acclimatisation societies to Australia, where they are now abundant. They have been so successful at colonising that they have become a pest and pose problems for native birds with whom they compete for nesting sites and food resources. Their communal and somewhat aggressive nature sees them killing other small birds, hatchlings and eggs and crowding food resources. This results in declines in certain species of small birds that find themselves in direct competition. However, all is not well with the sparrow. In many parts of the world including Europe, India, Eastern Asia and North America where sparrows are native, a disturbing decline in populations is occurring. It is estimated that in Europe alone there are now 247 million fewer sparrows than there were in 1980. This decline is now of growing conservation concern and further research is needed to better understand what is happening to this iconic little bird. The plight of the sparrow serves as a good example that conservation efforts are important to preserve all species and the worlds biodiversity, even those that may be a pest in some parts of the world.