My opinion is, yes, this could happen. Mantids sometimes grow quite large, and the fully grown female ones have a very strong grip (I remember this from direct experience when I was a kid...you can handle typical small mantids and they can't do a thing about it...but a really big one is a different story!) A very large female mantid could definitely kill a small hummingbird, and it would be a matter of a split second if the mantid decided to attack...they're extremely fast. They normally grab in such a way as to completely immobilize their prey (important in the case of wasps and such) and then they devour the head, killing the prey. Then they leisurely eat the rest of whatever unfortunate creature they have siezed. Mantids are attracted to motion, and they grab with one quick lunge. Sounds entirely plausible to me.