Two of the three ptarmigan species, rock and willow, live mainly in the Arctic. The third, white-tailed, also lives as far south as Colorado. (Psst! The “p” in ptarmigan is silent.) ...

7 crossings. Take the ptarmigan over. Return. Take the polar willow over. Return with the ptarmigan. Take the Arctic fox over. Return. Take ptarmigan over. Thus there are seven crossings ...