" Okay, forget Jennifer's questions, explain this answer! I'm not an engineer and never understood this answer, elaborate please! "
THANK YOU !
i suspect what we have here originally is an inaccurately posed question.
because : if the birds are sitting on the truck, and their little feets are pushing down on the truck, then the truck weighs what it weighs Plus the birds. so that there is the weight of the truck .. truck with it's birds. thusly : truck+birds=weight.
weight normally considered to be the amount of how hard something is pushing down. you need gravity (or close to it) to have weight in common terms. no gravity = no weight. and hence the moon-shot like term "weightless".
now, for us earthbound people, living as we do at the bottom of an ocean of air, if the birds take off and fly around inside the truck, or if the truck has open windows, or even if the birds are/were sitting in the back of a pickup, and are now flying all Around the truck (but not touching it), once the birds take off and are no longer pushing down (remember their little feets) on our truck. with flying birds, the truck will indeed weigh less if one is using the common and correct use of the term "weight".
but ... if what the original question meant to use was the was the word "mass", then that's a related, but different thing. because the mass of our truck+birds is the same regardless of if the birds are sitting, or flying, or even near the truck, their combined Mass will not change, nor will it change in the vacuum of orbital space, nor even if the entire lot is partially buoyant in our atmosphere.
much like managerial stupidity, mass is fairly durable stuff in the blue collar world. i assure you, that if peter attempted altering the mass of random objects within the sacramento city limits, if would almost immediately attract the attention of the Authorities. at any rate, being as it's california, no doubt i'd need a permit.