It's been reported that younger gulls initially try to open clams (the animal clams, not plastic containers) by pecking. I believe they learn the dropping-from-a-height tactic from observation, and suspect that the pecking at a plastic container is merely due to them not having seen it being open by a gull before. Also, worth considering that in their natural habitat, the only transparent things around (jellyfish) are squishy and stingy.