Where are the birds?
The birds are in the sky!
But seriously, it depends on the draw distance if you see birds or not, the most chance of seeing them is if you look straight at the sky, on a place that is located not too low, under clear day skies.
Also they're not always there, but added semi-randomly, and birds and dragons usually don't share the same territory.
Personally I sometimes only notice the birds because of their shadows when they fly over me.