A bird resting on the floor of an airtight box which is being carried by a boy starts flying. The boy will fell that the box is now:

In an airtight box, when the bird is sitting, the weight of the box is the sum of the weight of box and that of bird. When the bird starts flying the bird presses the air down to get upthrust. The reaction is on the bottom of the airtight cage. So that total weight is equal to sum of the weights of the box will feel the same weight whether the bird is sitting or flying in the airtight cage.