Southern Mexico’s offshore Pacific avifauna has been rather little studied. Murphy (1958) and Jehl (1974) reported observations from cruises that passed through Mexican waters in November and December 1956 and in April 1973, respectively. Pitman (1986) mapped the relative abundance of 57 species of seabirds in the eastern tropical Pacific on the basis of 4333 hours of observation between 1974 and 1984; only a small number of these 4333 hours (81 noon positions, 55 of which were off Baja California), however, pertain to Mexican waters (i.e., within 200 nautical miles of Mexican territory), and 74% of the 81 noon positions were between October and March (R. L. Pitman pers. comm.). Pitman’s atlas provides an excellent large-scale picture but lacks data on seasonal status of species, and the scale employed does not enable one to interpret local distributions. Other records of seabirds off western Mexico are widely scattered and mostly derive from nearshore land-based trips of a day or less (e.g., Binford 1970, 1989).
The Middle American Trench runs from the vicinity of the Islas Tres Marias, Mexico, to the Cocos Ridge, south of Costa Rica. The trench lies some 55–110 km (mean distance 75 km) offshore between Jalisco and Guerrero and is 15–50 km (mostly 20–30 km) wide. The trench is at least 3600 m deep, mostly 4300–4650 m deep from central Jalisco south, and increases to 5000–5200 m deep off central Guerrero; submarine mountains in the trench off Colima (Manzanillo) and Guerrero (Zihuatanejo) reduce depths to 3600 m. On either side of the trench waters quickly shallow to 2700–3200 m, and inshore the 1000-fathom (1800-m) contour line lies 20–55 km (mostly 35–55 km) off the coast.
From 28 April to 6 May 1992 we observed seabirds off western Mexico out to 100 km from shore, that is, from just offshore of to well inshore of the Middle American Trench, between the vicinity of Cabo Corrientes, Jalisco, and Acapulco, Guerrero. Here we describe the birds observed during our visit.