The simile "<u> they flock together like sheep </u>" simply means that, the recruits moves together in groups instead of moving separately during the war. This group movement exposes them to harms way leading the airmen to inflict maximum damage on them.
<em>On the other-hand, "</em><em><u>the wounded are shot down like hares</u></em><em>" suggest that, those who are wounded recruits still clusters together leading to the airmen to complete the task of killing them all.</em>
it might mean that frogs like being in public and "public" would mean that the frog doesn't like to be seen because it has camouflage. hope it helps ;)