Following an isolation gradient in West-Mexico the avifauna of three study areas is investigated, on mainland reserve and two island groups. The avifaunas are described and analysed regarding species richness, environmental attributes, endemism, broader ecological niches, and brood status. The taxonomical composition is determined on different hierarchy levels. Most species records come from mainland Chamela (262), followed by the coastal Marias (191) and the oceanic Revillagigedos (148). Thereof 139 bird species breed in Chamela, 61 on Marias and only 29 on the Revillagigedos. In total, in the three study areas 364 species were recorded, which belong to 22 orders. On island groups generally less taxa per hierarchy level are present than on the mainland, especially on the remote Revillagigedos. Here, on average 1.7 species are found per genus, 2.2 genera per family, and 2.5 families per order. In the analysis of the taxonomical hierarchy relations to each other an isolation gradient is identified: from mainland Chamela via the coastal Marias to the oceanic Revillagigedos the degree of relatedness decreases on average. The results support the theoretical hypothesis that on islands closely related taxa tend to exclude each other and that one generalist tends to replace several specialists. Small ecological niches are often not occupied by specialist species on islands, but are used by generalists.

Species richness, biodiversity, island biogeography, island theory, endemics, endangered species, conservation, Isla Isabel, Marias, Revillagigedo, Socorro, Mexican Pacific