History
Bird shifters, or rather, people turning into birds, is a rather common theme in mythology. Most famous of all is the story of Leda and the Swan, in which Zeus transforms into a swan to seduce Leda. Another greek story of people being turned into birds is the story of the Pierides, nine sisters who challenged the Muses and lost - then were turned into birds. Birds, like the notion of having wings and the ability to fly, are heavily associated with holiness, power, and creation. This is why most myths involve gods becoming birds or humans becoming birds due to divine intervention.
Bird shifters are very difficult to find information about, so we must refer to what we can observe biologically.
The largest flying bird was the Argentavis Magnificens, which weighed an average of 150-154lbs, and had a 21-24ft wingspan. This is well within human size.
One major point that people try to use to disprove or discredit the notion of bird shifters is that feathers are dead cells and cannot be shifted - but this is immediately undermined by the fact that fur and hair are also dead cells, yet many kinds of shifters (namely mammals) can shift fur with no issue.
So, with biological limits and what we've observed with other shifters supporting the idea of bird shifters, let's talk about how it would work.
Obviously, the shifter would stay the same mass when shifted, yet patterns as well as wing size and shape would vary depending on the type of bird they shift into, as well as the weight.