An iron bird is just a generic term for any sort of flying machine used in the modern age of Pangaea. Because most of the advanced technology and knowledge was lost during the war and continental divide, things such as fully functioning "airplanes" or "spacecraft" are still widely unused. This fact alone gives Moo's army a very big advantage when taking over the continents because they have the resources and power to build and maintain these large machines.
All sorts of iron birds are featured in the anime series, from the small hovercraft that Ed uses to escape captivity to the massive warships that Durahan uses to subdue the Eastern Continent. As Moo's reign of terror comes to an end, technology is allowed to advance further and flying machines become more commonplace, although still not available to the general public. The Searchers do make use of Most's private iron bird in season 3, as well as the Mocchi Smugglers.