These monstrous birds were probably more agile and less portly than previously thought. Imagine a flightless carnivorous bird three metres tall with a skull the size of a horse's, and a vicious, eagle-like bill. What you are imagining is a phorusrhacid ('terror bird'), an extinct lineage that includes the largest known birds. The discovery of an enormous fossil avian skull from the middle Miocene of Patagonia (Comallo, Argentina) prompts a reanalysis of what the phorusrhacids looked like, and the suggestion that there were marked differences between the morphology of large and small variants.