Bravery in this context is likely the same “brave” that is used in the phrase “O brave new world. That has such people in’t!” from The Tempest. It’s not “brave” as in courageous, but rather refers to handsomeness, beauty, splendor, etc. So Thoreau here is probably saying, “you go from the city into the country hoping to find something beautiful and all you find is a bunch of rodents.”
Bravery in this context is likely the same “brave” that is used in the phrase “O brave new world. That has such people in’t!” from The Tempest. It’s not “brave” as in courageous, but rather refers to handsomeness, beauty, splendor, etc. So Thoreau here is probably saying, “you go from the city into the country hoping to find something beautiful and all you find is a bunch of rodents.”