Betsy,
At this time of the year male hummingbirds are mostly concerned with mating, so they go where the females are. Females are mostly concerned with nesting, so they go where nesting habitat is. For Rubythroated hummingbirds, nesting habitat is mostly woodlands, not yards, unless the yard is near woodland. I guess your males moved off into the woods to court females. Another possibility is that some flowers have come into bloom nearby and your males moved to guard the flowers. It is possible that they are still within a mile or two of your yard and sometimes return to visit the feeders.

Ladyslppr,
Thank you for your input. I do live near woodlands. I am glad to know they are probably alright. I have had hummers for years and have never noticed this happening. Perhaps I have not paid too close attention and it has been going on forever.
Perhaps you could also tell me if hummers brood more than once a season in Michigan. I will post that question and perhaps you could enlighten me on this also.
Thanks again,
Betsy