Thoughts on the "caged birds in a plane" chestnut. Someone recently asked in the notes and queries section of a British newspaper, "would the weight of a plane flying at a constant altitude change if its cargo of birds in cages were all on their perches or flying about in their cages?". I have since discovered that this is a version of a textbook staple re Newton's laws and the respondants to the question duly answered that the flapping of the unperched birds would be transfered to the floor of the plane, hence no change in weight. However... 1) If the birds were moving horizontally around their cages as well as vertically (as flapping birds surely must) would not the small lift derived from pressure differences due to the shape/angle of their wings account for even a small proportion of their flight NOT being passed on to the plane, hence a small reduction in the weight of the plane? 2) Would the change in position of the plane's contents caused by the birds getting off their perches effect the plane's cumulative centre of mass and hence how its weight would be measured in terms of pull from the earth's centre of gravity? I appreciate that the whole thing is a thought experiment to demonstrate Newton and could never be practically measured but am interested to know if there is an official answer. Good conversation starter down the pub I've found.