Newton’s third law and conservation of momentum

Question

When a bird flies, it’s wings push air downward. As a result the air pushed back upward on the bird, which law of motion most applies to the way the wings of a bird beat?

Answer 1

Based on the description, Newton's Third Law is the most appropriate: there are various ways of expressing it, the most famous (and maybe also a little vague) "for every action, there is an equal and opposite reaction".

One other way of describing it is; when an object exerts a force on another object, the other object will exert an equal and opposite force on the original object. One example is having a box sitting on a table. The table will exert an upward vertical normal contact force on the box. The box in turn will exert a downward contact force on the table. The magnitudes of the mutual forces will be equal.

In the example here, we have a bird on a downward portion of beating its wings exerting a downward force on the atmosphere just below. The atmosphere would exert an equal and upward force on the bird.

(Forgive my going on a tangent below, but it might be of interest to look at, further down the road)

Now the motion that allows a bird to fly is actually a little more complicated and involves an understanding of aerodynamics, (and goes beyond the scope of this question) which birds grasp, because they're smarter than us.

The shape of a bird's wing is particularly important the upper portion of the wing is actually longer that the lower portion; this will cause the air flow just above the wing to be faster than that of the air flow below the wing. This will cause the air pressure above the wing to be lower that that below the wing and will create a net force upwards. Bernoulli's Principle describes this.