Size and Weight:
More often than not, paleontologists can infer a dinosaur's behavior from its anatomy. That's the case with Mononykus, whose small size, long legs, and long, curved claws point to it being an insectivore that spent its day clawing at the Cretaceous equivalent of termite mounds. Like other small theropods, Mononykus was probably covered in feathers, and represented an intermediate stage in the evolution of dinosaurs into birds.
By the way, you may notice that the spelling of Mononykus isn't quite orthodox by Greek standards. That's because its original name, Mononychus, turned out to have been preoccupied by a genus of beetle, so paleontologists had to get creative. (At least Mononykus was given a name: discovered way back in 1923, its fossil languished in storage for over 60 years, classified as belonging to an "unidentified bird-like dinosaur.")