How much thrust can a bird create in flight?
Well, obviously, most flying birds are able to generate at least
as much thrust as is their body weight. But most birds are unable
to take off in vertical flight - so that modest thrust is about the
limit for most birds. Indeed, few birds can manage vertical flight
with a sustained ascent.
In New Zealand, we have a native pigeon, Kereru, which loves a
matai berry - about the size of an acorn. These they eat in
sufficient quantity such that they cannot even maintain level
flight!