Possible Clues:

• Zeroing (in on)
• Like some pigeons
• & 22 Trained bird
• Set off something for the pigeons?
• (Of a weapon) able to find and hit its target
• One of those high-flying racers leaves base for a round-trip mission
• One of those living in pigsty takes in barest minimum of those types returning with the messages
• Pig swallows minimum of those types returning with the messages
• Pigeon's activity, perhaps
• This pigeon always comes back