BREEDING BIOLOGY OF APLOMADO FALCONS IN DESERT GRASSLANDS OF CHIHUAHUA, MEXICO

We located two populations of endangered Aplomado Falcons (Falcofemmal is) in desert grasslands in north-central Mexico in 1992. During sprin g 1993, we quantified breeding success, habitat use, and prey selectio n by the easternmost population, which is located 40 km from the Unite d States. We captured 17 individuals (one subadult male, seven adult m ales, three subadult females, six adult females) and placed tail-mount ed radio transmitters on 15 of them. Females weighed 50% more, had tai ls 9% longer, and had wing chords 11% longer than males. Breeding seas on ranges of six radio-tagged individuals monitored more than 100 d ra nged from 3.3-21.4 km(2). Woody plant density ranged from 11.2-139.5/h a and percentage of ground-cover ranged from 28.9-69.5% in ten falcon territories. Six of seven nests were in yuccas (Yucca elata, Y. torrey i). Seven nesting pairs had a mean clutch size of 2.6, mean hatching s uccess of 1.6, and a mean fledging success of 0.6. We identified 87 pr ey items from pellets, prey remains, and observed successful hunts; 82 were avian and 5 were insect. Meadowlarks (Sturnella spp.) were most abundant on potential prey transects and in the diet of Aplomado Falco ns. Brown-headed Cowbirds (Molothrus ater), Greater Roadrunners (Geoco ccyx californianus) and hummingbirds (Trochilidae) were identified in diets, but were not detected on transects.