Size and Weight:
In most cases, the similarity between small theropod dinosaurs and birds is limited to their size, posture, and feather coats. Nomingia took its birdlike attributes one step further: this is the first dinosaur ever discovered to have sported a pygostyle, that is, a fused structure on the end of its tail that supported a fan of feathers. (All birds have pygostyles, though some species' displays are more garish than others, as witness the famous peacock.)
Despite its avian features, Nomingia was clearly more on the dinosaur than on the bird end of the evolutionary spectrum. It's likely that this dino-bird used its pygostyle-supported fan as a way of attracting mates--the same way a male peacock flashes its tail feathers to reel in available females.