The smallest non-avian dinosaurs weighed a little less than a pound—around the size of a bunny. Weighing in at less than 2 grams, the bee humming bird would be dwarfed by the smallest non-avian dinosaur. “If we want to understand how today’s biodiversity evolved, we need to understand why tiny dinosaurs appear to have been missing.” They used mathematical models estimating the body size of organisms based on how efficiently their physiologies convert energy into offspring. Instead of physiological restrictions on size, they determined non-avian dinosaurs were limited by ecological restrictions, like competition, predation, and the availability of niches.