Most bird baths are actually two pieces, a basin sitting loose on top of a pedestal, held in place by nothing more than gravity and its own weight. Separate them and each half becomes a manageable two-person lift instead of one wobbly, top-heavy load.
Pro tip: If your bird bath basin has a rough, uneven bottom from age, pack a folded towel underneath it in the truck so it sits flat and does not rock during transit.
Watch out: Never carry a bird bath basin by its rim alone; the thin edge is the most likely place for a chip or crack, especially on cast concrete models.