This EBA comprises the coastal plain in the north of Mexico from the mouth of the Colorado river (at the head of the Gulf of California), south through the states of Sonora, Sinaloa, Nayarit, Jalisco and Colima; it also includes the Tres Marías islands located some 150 km off the coast of Nayarit. The upper limit of the EBA is c.1,200 m and within these altitudes there is a variety ofhabitats, the main ones being tropical deciduous (dry) forest, thorn forest, thorn scrub, semi-humid subtropical forest and mangroves. Immediately inland ofthe EBA lies the Sierra Madre Occidental and trans-Mexican range (EBA 006),while the southern end abuts the western end of the Balsas region (EBA 008 in part).

Most of the EBA's restricted-range species occur throughout it, but there are some variations in this pattern. Cyanocorax beecheii and C. sanblasianus have nearly allopatric ranges with perhaps a small zone of overlap in the region of San Blas in central Nayarit. Indeed the southern part of the range of C. sanblasianus in Michoacán and Guerrero falls outside the boundaries of this EBA as none of the other species occurs this far south. Only Forpus cyanopygius and Turdus graysoni occur on the Tres Marías islands, and the status of T. graysoni on mainland Mexico is unclear as it has been recorded on the Nayarit coastal plain opposite the Tres Marías islands between December and June (Howell and Webb 1995a) but, aparently, not in other months.

There are few differences in the general habitat requirements of the species, as the majority are found in tropical deciduous forest. However, Thalurania ridgwayi prefers more humid forest than the other restricted-range species of the EBA, and so tends to occur in shaded barrancas within the foothills of the coastal plain which are patchily distributed within Nayarit, Jalisco and Colima states. Although described originally as a new species, it was treated as a subspecies of Crowned Woodnymph T. colombica until a revision of the taxa by Escalante-Pliego and Peterson (1992) showed that it was better treated as a distinct species.

The dry forests of western Mexico have largely been ignored as a key habitat for biodiversity conservation, with no comprehensive plan to conserve them (Ceballos and García 1995). Much of the coastal plain within this EBA is densely populated and consequently the dry forests in particular have been heavily degraded; many of the more humid forests in the barrancas of the foothills have been turned over to coffee production.

Fortunately, most of the restricted-range species seem tolerant of degraded habitat and so only Thalurania ridgwayi is currently classified as threatened (Vulnerable).It is apparantly locally common within its shaded barranca habitat, but it may well prove to be threatened by habitat destruction when its range and the causes of its patchy distribution are better understood (Collariet al. 1994).

Two widespread but threatened species also occur in this EBA: Yellow-headed Parrot Amazona oratrix (Endangered) occurs in the southern part of the EBA, with an important population (the endemic tresmariae) on the Tres Marías islands; and Black-capped Vireo Vireoatricapillus (Endangered) winters partly within this EBA. There are few protected areas in the EBA, but encouragingly a new Biosphere Reserve, Chamela-Cuixmala (covering 131km²), wasdecreed in 1994 to safeguard dry forests of western Mexico. There are three other large protected areas in the region, embracing mainly highland and humid forests: Cerro San Juan Special Biosphere Reserve, Nayarit (270 km²); Sierra de Manantlán Biosphere Reserve, Jalisco- (1,396 km²);and Nevados de Colima, Jalisco-Colima (222 km²). The seven Key Areas for threatened birds identified by Wege and Long (1995) in this EBA cover the above protected areas and three other sites,including the Tres Marías islands.

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: North-west Mexican Pacific slope. Downloaded from https://datazone.birdlife.org/eba/factsheet/5 on 23/11/2024.