Further affects of air resistance on small birds
For small birds the drag produced by their wings has a bigger effect than just the flex of their wings, it effects their whole pattern of flight, below a certain size, birds will abandon a conventional flap/glide pattern and instead flap in short bursts, then pull their wings in completely for a time. This is called a bounding flight pattern. The small bird will still glide but only when it wishes to decelerate thus using the drag its wings produce. Here is an example of bounding flight -
Each dot on this graph represents a species of bird, the larger crosses represent the birds which use a bounding flight pattern. We can see that as a rule, a bird whose mass is below 100g will tent to adopt bounding flight instead of a flap/glide pattern.
And these diagrams show the different paths of the bird (yellow) and the wing tips (orange) of the two flight patterns. Flap-gliding flight path (left) and flap-bounding flight path (right).
I've started experimenting with this idea for the character of Gylfie I believe it may be a interesting way to accentuate her small size.
As with many aspects of bird flight, few things are black and white. An interesting blend between the two flight patterns is used by this cockatiel. It doesn't bound as such, instead flexes it's wings and uses an accentuated up-stroke
Also I wouldn't use hummingbirds as reference for small bird flight unless it is specifically a humming bird you're animating, they way they fly is quite unique to them and not all that relevant to owls and other bird flight.
So, in summary to show a bird's scale I would look to the following variables
* flap speed - a smaller bird has a faster flap cycle