Great Bustard is one of the two heaviest flying birds (and animals) in
terms of generic males, the other being the Kori Bustard.
The Great Bustard has lost much of its former range but is
still native to isolated parts of southern and Eastern
Europe and western/Central Asia, southern Siberia (the largest
population) and China. It is being re-introduced in the wild to the UK.
Males (above) are significantly larger
than females, taller with substantially bulkier necks and
bodies and may be twice as heavy. A typical male is some 10kg
but the heaviest can be closer to 20kg. Individual male
Dalmation Pelicans and Mute or Trumpeter Swans may be heavier than
individual male Great Bustards but generically the male
Bustards are heaviest.
Females, above, are more slender with
slimmer necks, but the same cryptic plumage, white body and
grey head/neck. (The name "Bustard" comes from the Latin for
the bird: "avis tarda").
Unfortunately, I don't yet have a photo of
the male's amazing courtship posture and dance in which he
fluffs out his moustache and neck feathers while withdrawing
his head and body into a bundle of white, raised undertail
feathers but the female above seems to be doing a (much less
impressive) dance of her own.