According to the law of conservation of linear moment um we know that the linear momentum of a system of constant mass remains unchanged in the absence of any external force. Now treating the bird, container and the air in the container as the system of constant mass we find that the force which the bird exerts in flying is an internal force.But as by an internal force the momentum of a system cannot be changed,so the weight of the system will not change, i.e., we cannot tell when the bird is flying or sitting by weighing the container.
