Animal Architects (click to download - subscription required)
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This is an interesting unit. Its a kind of a variation on the 4th grade Zoology block with a Physics twist. I originally got the idea when a friend needed to include some Physics for the Canadian requirement for her child's grade level. It might be helpful to someone else as well!
By the way, while looking for the Encyclopedia Britannica book/tape set Prairie Dog Town (I didn't find it), I found a book by L. Frank Baum (creator of the Wizard of Oz) called Prairie-Dog Town Languishing in obscurity for many years, it is now back in print thanks to Addison Press. Fascinating! I'd love to read it.