The island of Raso is primarily flat, with some small peaks in central-northern parts of the island, which rise to a maximum of 164 m. The entire south-western area is a flat, rocky plain, parts of which are covered with large boulders. Dry riverbeds cross the island. There is little vegetation except for a grassy area in the south-west. The coastline is entirely composed of rocky cliffs.
See Box and Table 2 for key species. The most famous bird of the island is, without doubt, the endemic Alauda razae, which is entirely confined to Raso and has a total population of c.45 pairs. The species roams all over the island, but breeding is largely confined to the grassy area in the south-west. Calonectris (diomedea) edwardsii is the most numerous seabird on the island, with thousands of pairs breeding on the cliffs along the southern shore and more breeding inland in holes under boulders. Other breeding procellarids are Puffinus (assimilis) boydi, Bulweria bulwerii and Oceanodroma castro. There is a colony of c.125 pairs of Sula leucogaster and 25–40 pairs of Phaethon aethereus, numbers which are only a shadow of those recorded c.100 years ago. Egretta garzetta, Pandion haliaetus, Falco (tinnunculus) neglectus, Tyto (alba) detorta and Passer iagoensis also breed.
Non-bird biodiversity: The endemic giant skink Macroscincus coctei is thought to have become extinct early in the twentieth century (see site CV010). The endemic geckos Tarentola caboverdiana and T. gigas still occur in good numbers. Other lizards include Mabuya stangeri and Hemidactylus bouvieri.
BirdLife International (2019) Important Bird Areas factsheet: Ilhéu Raso. Downloaded from http://www.birdlife.org on 21/01/2019.