Technical Abstract: The ecological interactions between grasshoppers, predators, climate conditions and plants that can limit the population growth of grasshoppers are poorly understood. A number of field experiments have generally shown decreased grasshoppers densities in the presence of avian predation, indicating that top-down control by avian predators can regulate grasshopper populations in some systems. Avian predator limited sites tend to have low population densities with few large body size grasshoppers. However, the effects of avian predators on grasshopper populations can differ between years and between sites in the same habitat. I conducted experiments examining grasshopper populations in avian exclosures and control plots at two locations in eastern Montana, USA. Avian predation did not have consistently strong top-down effects on grasshopper densities, species richness or species diversity at the two sites. The effects of predation varied among years and between the two sites. Bird predation led to a reduced percentage of medium body size grasshoppers in control plots exposed to birds in most years. Therefore, birds modified species composition through size selective predation even in years when they did not reduce overall grasshopper densities. Bottom up factors and abiotic conditions may play an important role in the population dynamics of grasshoppers at these sites.