On (1) No. Not if they are steadily descending (zero vertical acceleration) and I think we both mean that by "gliding down." (2) If flying without vertical acceleration their wings have a upward force on them equal (average over wing cycle) to their weight and all of them (their body) has a downward gravity force equal to the weight. I.e. net force F = 0 = ma so a = 0. AS STATED by "if condition" at start of this reply. If a (vertical) is not zero there is a change in the force on floor from when birds are just standing on it. There may be lots or no lateral force - it does not affect the scale weight. If there is net lateral force on the truck walls made by the birds, (not equal opposite on both) the birds have a horizontal acceleration. (3) Definitely not agreed. Only if the bird is in "free fall" (has g acceleration towards the center of the Earth like an astronaut) does his weight disappear. (4) No agreement. We only need consider the downward force flying bird applies to the air. IFF it is equal to his weight then bird has no vertical acceleration and his weight shows fully on the scales (with that of the truck) exactly as if he were standing on the floor (another zero vertical acceleration case.) Have some faith in Newton's laws especially the first: "A body at rest ... or in motion ..." (5) While door is open (and flying bird not too deep inside the truck) the downward motion it gives to the air may be partially stopped by truck floor and partially stopped by ground behind (and under) the truck. As doors are shut more is stopped by truck floor. (6) Quickly closing doors may compress some air inside the truck and make it heavier until the extra air leaks out. Even before the bird enters the truck some of the downward air flow is probably being stopped by truck floor. The full weight of level flying birds is stopped / supported by something all the time. As noted earlier if truck is very tall and bird is near ceiling that "stopping" could be by wall friction with the walls then pushing down on the floor. THESE DETAILS ARE NOT IMPORTANT, stop being concerned by them. Only the vertical F = ma matters and I have several times explained how. I will not again.