LAUSR

Abstract. The breeding distribution and population size of the Red-billed Tropicbird (Phaethon aethereus) in Mexico were reviewed. This species has been confirmed to breed on 14 and potentially breed on six Mexican Pacific islands: two in the North Pacific, 12 in the Gulf of California, and six in the Tropical Pacific. The breeding population has been estimated to be approximately 1,901–2,725 breeding pairs. This estimate includes between 507–713 pairs in the Gulf of California, 1,391–2,004 pairs in the Tropical Pacific, and the rest in the North Pacific. The largest colonies are Peña Blanca in Colima (1,250–1,650 pairs), Farallón de San Ignacio in Sinaloa (150–228 pairs), San Pedro Mártir in Sonora (150–190 pairs), San Benedicto in the Revillagigedo archipelago (50–200 pairs), Isabel in Nayarit (87–155 pairs), and Morros El Potosí in Guerrero (100 pairs). The portion of the population that breeds in Mexico is numerically important at both regional (Pacific Ocean) and global scales, representing approximately 50% of eastern Pacific breeding pairs and 25% of the global population, respectively. Therefore, protection of these colonies can play an important role in the global conservation of this species.