>>Why would they go home, if they were short-lived? Colonists, etc. would not need to return home. If they had longer lifetimes it would not matter that they were gone a few hundred. LOTS of SF written about the effects of time-dialted travel on civilizations and people.

Sorry Bruce, you seem to have missed part of the conversation.
We're talking about the alleged aliens visiting Earth in UFOs please note the thread title.

Aside from the behavior of John Tesh and Tom Cruise, I see no evidence of Alien colonization of the Earth.