The ordinary meaning of nap is to doze off during the day. The secondary meaning is to be offguard. Hence the phrase Catch them napping . Flying by the nap of the earth refers to the practice of flying low to the earth (undetected by radar) to catch the enemy by surprise (or napping).
the tribunes The Tribunes were elected representatives that protected the plebeians from mistreatment. The Tribunes attended meetings of the consuls or Senate in an attempt to bring justice to the plebeians.
Ducks need to be kept in a secure pen or house when you are not at home that will protect them from predators. To be secure, housing needs to have solid sheeting or welded mesh (with wire at least 1.2mm thick) on the roof, floor and walls. Provide as much space as possible for each duck.