Size and Weight:
Discovered in 1903, Ornitholestes was given its name (Greek for "bird robber") by the famous naturalist Henry F. Osborn before paleontologists had grappled with the evolutionary origin of birds. It's certainly possible that this slender theropod preyed on the proto-birds of the late Jurassic period, but since birds didn't really come into their own until the late Cretaceous, it's more likely that Ornitholestes feasted on small lizards and the carrion left over by larger carnivores. Whatever the case, there's not much fossil evidence to support either supposition: unlike the situation with its close cousins Coelophysis and Compsognathus, remains of Ornitholestes are few and far between, necessitating a large amount of guesswork.
Ornitholestes' reputation as a bird-eater has much in common with Oviraptor's reputation as an egg-stealer: these were inferences drawn on the basis of insufficient knowledge (and in the case of Ornitholestes, the myth was perpetuated by a famous painting by Charles R. Knight depicting this dinosaur preparing to eat a captured Archaeopteryx). There's still a lot of speculation about Ornitholestes: one paleontologist suggests that this dinosaur snatched fish out of lakes and rivers, another maintains that (if Ornitholestes had hunted in packs) it might have been capable of taking down plant-eating dinosaurs as big as Camptosaurus, and yet a third believes that Ornitholestes may have hunted by night, in a deliberate attempt to avoid (and outfox) its fellow theropod Coelurus.