I'm trying to find a series of books that I read in 1987. I don't know the title, who the author was, or even who published it, they were aimed at teenagers. The plot was about earth being invaded by aliens in the future. Energy beam weapons have no effect on the aliens as they have force shields, but a group of soldiers discover a stash of convention weapons in a museum that penetrate the force fields. This group of resistance fighters are earth's only hope blah blah. I seem to remember they steal a space ship that resembles a swan.. or some other kind of bird. Sounds all really corny but I never actually finished the last book and it's eaten away at me for twenty years....I need closure

Can anyone help?