Nesting Ecology of Thick-Billed Parrots

Abstract

-During a four-month search for nesting Thick-billed Parrots (Rhynchopsitta pachyrhyncha) in 1979, we found 55 active nests at elevations of 2,300 to 3,070 m in northwestern Mexico's Sierra Madre Occidental. All nests were in cavities of live or standing dead (snags). Pine (Pinus spp.) snags contained over one-half (58%) of these nests. These parrots laid two to four eggs (X = 2.9; SD = 0.65) between mid-June and late July and young flew from their nests between early September and late October. Nesting density was apparently related to availability of suitable cavities and was variable, with some nests as close as 2 m apart in the same tree. The nesting season corresponded with the maturing of pine seeds, the parrots' principal food. Commercial logging of live pines for lumber, and of pine snags for pulpwood, is eliminating large numbers of proven and potential nest sites. Forest management practices should be modified to leave some suitable throughout the forest as potential nest sites for Thick-billed Parrots. Thick-billed Parrots (Rhynchopsitta pachyrhyncha), known locally as guacas or guacamayas, inhabit the highland pine forests of the Sierra Madre Occidental in northwestern Mexico from the states of Sonora and Chihuahua south to Jalisco and Michoacan (Blake and Hansen 1942, Marshall 1957, Schnell et al. 1974). Formerly they ranged north into the forested mountain islands of southeastern Arizona and southeast to the Mexican state of Veracruz (Ridgway 1916, Wetmore 1935). These parrots breed in the northern part of their range and winter mostly on the high volcanoes in the southern part of their range (Thayer 1906, Schnell et al. 1974). They nest in tree cavities and feed primarily on pine seeds (Forshaw 1978). The U.S. Fish and Wildlife Service (1980) listed the Thick-billed Parrot as endangered, and the International Union for Conservation of Nature and Natural Resources considered it vulnerable (King 1977). The parrot was included in Appendix I of the Convention on International Trade in Endangered Species of Wild Flora and Fauna (U.S.F.W.S. 1977), which implemented strict international trade restrictions for the species. Concern for the species has arisen owing to its near total absence from the United States since 1935 (Phillips et al. 1964), an apparent decrease of reports of it from Mexico, and increased logging and human population in the parrots' range (Monson 1965). The nesting ecology of these birds has never been studied in detail, despite their relatively wide distribution in Mexico, their proximity to the United States, and international concern for their welfare. Thayer (1906) and Bergtold (1906) published descriptions of 10 nests and their contents found in northwestern and central-western Chihuahua. Friedmann et al. (1950) reported nesting in southwestern Chihuahua, but gave no details. Bailey and Conover (1935) reported that Brock taken young from cavities in trees in the state of Durango. Our goal was to gather information on the status of breeding Thick-billed Parrots and their habitat. STUDY AREA AND METHODS For 92 days between mid-April and early October 1979, and for the first week of September 1980, we searched for nesting Thick-billed Parrots. Our study area was the conifer forests of the Sierra Madre Occidental in northwestern Mexico from northwestern Chihuahua (108030'W, 30020'N) south to central-western Durango (105030'W, 2000'N; Fig. 1). The Sierra Madre Occidental is a range of rugged mountains extending from northwest to southeast. It is 100 to 200 km wide and 1,200 km long with many ridges above 3,000 m (elevation above sea level) and a few peaks rising above 3,300 m. Large rivers drain to the west, dissecting the range with valleys and canyons often 1,000 to 2,000 m deep. The conifer forests were mostly above 2,000 m elevation and consisted primarily of Arizona pine (Pinus arizonica), Mexican white pine (P. ayacahuite), and several species of oaks (Quercus spp.). Northern exposures of the higher ridges also had Douglas fir (Pseudotsuga menziesii), quaking aspen (Populus tremuloides), and fir (Abies spp.). Chihuahua pine