Variation in the diet of greater roadrunners (geococcyx californianus) in a mesquite-thornscrub ecosystem

Greater roadrunners (Geococcyx californianus) are conspicuous and well-recognized members of the southwestern fauna. Conventional wisdom suggests that roadrunners prey heavily upon vertebrates, particularly lizards and snakes, but detailed studies are lacking. We investigated roadrunner diet at a site in southern Texas inhabited by four state-threatened reptiles to better characterize seasonal variation in diet and speculate on the relative impact of these predators on state-threatened species. We examined gizzards from 144 greater roadrunners collected across four seasons (Summer 1997, Spring 1998, Summer 1998, Winter 2002) and enumerated the number of food items for each taxa identified. We compared dietary richness, diversity, evenness, number of food items, and similarity among the four seasons. We also evaluated the relationship between roadrunner body size and diet, and we estimated annual consumption rates for each taxa found in the diet. Roadrunner diet was rich and highly variable, but 95.9% of 6,834 identifiable food items were invertebrates. Overall, grasshoppers and beetles dominated all samples. Total number of food items consumed, particularly weevils, birds, and fruit, increased during the winter sampling period. There was a significant, but weakly predictive, inverse relationship between wing chord length and dietary richness. Our calculations suggested the average roadrunner would minimally consume 34,639 prey items per year on this site. Snake consumption was less common than the popular perception of greater roadrunners would suggest. The consumption of vertebrates seemed to coincide with general abundance or when weather patterns decreased abundance of invertebrates. Consumption of two state-threatened species (Texas horned lizard [Phrynosoma cornutum] and Texas indigo snake [Drymarchon melanurus]) occurred, but not at a level to suggest that greater roadrunners negatively impacted populations of these species. Thus, the diet of this species appears to be driven by availability, and greater roadrunners act as exceptional generalist omnivores.