This is what you do:
buy a regular kitchen gram scale. i got mine on ebay for about $30, including shipping. it came with a plastic bowl so i can weigh babies easily, and it can weigh stuff up to 5000g. its digital and works just as good as the expensive scales.
at the zoo when we weigh smaller, flighted, un-tame adults, we put them in a little plastic basket like this:
http://www.cometplastics.com/gifs/yellow-basket.jpg
Just put the basket on top of the scale and tare it. then open the top of the basket and put the bird in, let them settle down. you get the birds weight without having to add or subtract anything :)