Dr. John A. Wiens on Competition, Scarcity, abundance, and Plenty
From www.scienceblogs.com/grrlscientist and BIRDCHAT: Tittle 17, US Code, Copy Write Law may apply.

Examining grassland species with graduate student Mr. J.T. Rotenberry, Dr. John A. Wiens, avian community ecologist, found that in periods of abundance of food, species did not compete for resources, while in cases of scarcity, they did. applied to feeders in winter, dominant species and individuals win battles for feeder perches by asserting their alpha statuses. Larger species do not always dominate. In 17 out of 48 cases- 35.4%- Black Vultures supplanted TV's at carcusses* (Stewart 1978). Dominant immature ravens suppressed subordinants by "yelling"(Heinrich & Marzluff 1991).

* r= -0.46, p= 0.05

Heinrich, B., & J.M. Marzluff. 1991. DO COMMON RAVENS YELL BECAUSE THEY WANT TO ATTRACT OTHERS? Behav. Ecol. Soc. Biol. 28: 13-21.

Stewart, P.A. 1978. BEHAVIORAL INTERACTIONS AND NICHE SEPARATION IN BLACK AND TURKEY VULTURES. Living Bird 17: 79-84.