The Add-On, produced by Andrew Duff and Ann Lawson for the exclusive use by BirdBase and BirdArea, gives these programs the ability to handle the world's more than 4500 mammal species in exactly the same way that they handle the world's bird species. The Add-On's installation utility creates a new folder on the hard disk. Next a copy of BirdBase is put in the new folder, but with a list of mammal species and families instead of the list of bird species and families. Then the same is done for BirdArea and its EditData utility, but with mammal ranges instead of bird ranges. And then desktop icons are installed for the BirdBase and/or BirdArea plus EditData programs used for mammals, with labels that distinguish them from the icons for the programs used for birds. Henceforth the bird icons start the programs when birds are to be treated and the mammal icons when mammals are to be treated.