air downwards, which must exert a force on the ground beneath. However, not all of this force reaches the ground: a lot of the moving air's kinetic energy is lost in turbulence, sending it sideways or even upward; still more will be lost to sound and heat. So the total force on the ground (or bottom of the lorry) is less than the force that the hovering rocket is throwing out. Of course, birds flap their wings so their downthrust isn't constant. When a bird's wings are swinging upward relative to the air, they're not sending any force toward the bottom of the lorry. If you could train the birds to all flap up as you crossed the bridge's weak point, they'd all effectively be in free-fall for a moment. For this brief moment, no bird would be pushing any air downwards to exert force on the bottom of the lorry. This would make them weightless for the purposes of this question. In a less extreme version, if they were all gliding downwards then the down force generated by the angled wings moving through air is still less than equivalent to their weight. Finally - and more easily than training the birds - you could have a wire mesh floor. This would allow descending air to pass through the lorry bottom unhindered. The birds' flapping would therefore not transfer any downward force to the lorry, but would still be transmitted to the weak bridge. Hopefully though, the extra distance between