Boa constrictor occidentalis is the southernmost subspecies distributed from 30 º N to 36 º S and is the only member of the genus Boa present in Argentina. It is found in extreme climates in the semiarid plains of the Phytogeographic regions of Chaco, Monte, Espinal of Argentina. For central-eastern of San Juan Province this subspecies occur in Chaco Árido. B. constrictor occidentalis is a “threatened” species. Here we report a non-interrupted for observation predation case of B. constrictor occidentalis attacking from ambush.On 15th December 2008, we observed in Sierra de Guayaguas, 25 de Mayo Department, corresponding to Chaco Árido Phytogeographic Region in Argentina, a free-ranging adult individual B. constrictor occidentalis (total length three meters aproximatelly) using sit and wait foraging mode, lurked beneath the canopy of Ximenia americana (albaricoque) shrub near to trail of birds. Later, an individual of “chuña patas negras” Chunga burmeisteri Cariamidae that was passing by this trail was caught by a fast attack, biting and constrained until die. C. burmeisteri was ingested completely during approximately 25’ minutes including suffocation and deglution stages. This observation suggests that B. constrictor lurk passively in places when “walking birds” feed shrub’s fruits. This may be different for small mammals typical of these environments, where the active pursuit on burrows is the main Boa foraging mode.This note constitutes a first record of predation C. burmesteri by B. constrictor occidentalis suggests that and attack from ambush mode play a major role on predation of “walking birds”.

Fil: Laspiur, Julio Alejandro. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Departamento de Biología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan; Argentina

Fil: Acosta, Juan Carlos. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Departamento de Biología; Argentina

Fil: Blanco, Graciela. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Departamento de Biología; Argentina

Fil: Marquez, Justo. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Departamento de Biología; Argentina