The Red-billed Tropicbird (Phaethon aetehreus) is a seabird that breeds on islands in tropical seas. The Mexican government has listed it as threatened; however, the International Union for Conservation of Nature considers it of least concern, although it recognizes that its global population is decreasing. Its breeding ecology was studied at Peña Blanca island, Colima, México, through monthly visits from 15 January 2008 to 10 December 2009. By extrapolating density in a 4,227-m² survey area, 1,390 (2008) and 1,225 (2009) nests were estimated for the entire colony. In both years, the earliest eggs were laid in September, and the last active nests were recorded during 14 May–19 June, with peak activity on 7 March 2008 and 25 February 2009. Egg volume was not different between years (57.4 cm³ ± 5.1 vs. 56.8 ± 3.3), but nest success was higher in 2008 (77% vs. 57% in 2009). Highest nest densities occurred between January and May, coincident with regional upwelling processes (colder water and high concentration of nitrites, nitrates and chlorophyll-a). The Red-billed Tropicbird colony on Peña Blanca is the largest Mexican colony and one of the two largest colonies of the world.