Birds were once what everyone knew of them as: beasts and pets to be kept in cages or left to roam, hardly more intelligent than a dog or rat. But then a deadly virus known as A(H5N1) appeared, causing an especially bad outbreak in 2068. It was carried by avians, and initially only caused harm to them. In time, it mutated and began to spread to humans at what was apparently an alarming rate. More than that, it was exceptionally deadly in humans. In an attempt to save themselves, humans constructed a virus in an attempt to cull the avian population and prevent the A(H5N1) virus from becoming pandemic. This was Operation Carneades and it happened in 2070.

However, instead of killing birds, the new virus caused rapid mutation and evolution of avians. Beginning with doves and spreading to other birds, the virus caused increases in intelligence and eventually size until they mutated into something more akin to myself and Okosan--that is, capable of speaking and using tools with our wings and other things like that.

The gambit of humans failed. A large number died out from A(H5N1). In the latter half of 2071, birds began to raid human settlements in what is called Hitchcock's Winter. But 2100, when the war ended, the human population was at 4.6% of its original estimate. Following a truce in order to preserve what remained of humanity, legally humans were segregated and disallowed property and to live in avian cities.

For the record, the year where I am from is 2188. Just as a frame of reference.