The heaviest flying bird is the Mute Swan (Cygnus olor), which can weigh up to 33 pounds (15 kilograms) on average. However, it's worth noting that the weight of birds can vary significantly across species. Some large flightless birds, such as the ostrich and the emu, weigh much more but are unable to fly. The ostrich (Struthio camelus) holds the title for being the world's largest bird, with males weighing around 220-350 pounds (100-160 kilograms) and standing over 9 feet (2.7 meters) tall.