Which of these is one of the heaviest living flying birds?

The great bustard ('Otis tarda') is a bird in the bustard family, the only member of the genus 'Otis'. It breeds in open grasslands and farmland from northern Morocco, South and Central Europe, to temperate Central and East Asia. European populations are mainly resident, but Asian populations migrate farther south in winter. It has been listed as Vulnerable on the IUCN Red List since 1996.

Portugal and Spain now have about 60% of the world's population. It became extinct in Great Britain when the last bird was shot in 1832. Recent attempts to reintroduce it into England have met with some success and there is a population of 40 birds on Salisbury Plain, a British Army training area. Here the lack of public access allows them the freedom needed as a large ground-nesting bird.

The huge adult male great bustard is amongst the heaviest living flying animal.