Then the 18-55 will be just fine
Be sure to take seeds/bread crumbs/bird food then. For settings, I'd avoid the lens wide open, as the edges are a bit soft. This may mean raising the ISO for a decent shutter speed. Experiment as much as you can! Try to get a shot of your gf with a bird eating out of her hand. She has to stand/sit very still with bird food in her open palm, but they will eventually come if they're as tame as you say they are. Keep an eye out for good landscapes
And (this is just a quirk of mine) I always try to shoot animals in their natural surroundings. So don't be afraid to include some trees/bushes. Spare batteries and extra memory cards are always good, and you might consider a light tripod
EDIT: Some birds I've seen at a mountain nearby are so "trained" that they'll snatch a piece of bread in mid air if you toss it up. You might try it for a shot of a bird in flight! Those are always cool, if the shutter speed is high enough to freeze the wings