It depends on what you mean by ‘big’. The heaviest flying creature alive today is the kori bustard, weighing in at 18 kilograms (40 pounds) and with a wingspan of 2.8 metres (nine feet). However, it is not generally considered the largest. That title typically goes to a South American vulture called the Andean condor. Males can weigh as much as 15 kilograms (33 pounds) and have a wingspan of up to 3.2 metres (10.5 feet).
Both these birds are tiny in comparison to some of the flying creatures of the past. The now extinct Quetzalcoatlus northropi, a reptile that lived 68 million years ago, had a wingspan of 10.4 metres (34 feet) and weighed between 200 and 260 kilograms (441 and 573 pounds).