PETER THEOBALD says:
A flightline is simply the route a pigeon takes when it leaves its roosting wood and heads out to the place it has chosen as a feeding ground.
It is possible to watch a known roosting wood, preferably around midday, and follow the line of any emerging woodies to discover where they may be feeding.
Be prepared to follow them perhaps four or five miles (not always easy, when pigeons don’t stick to the main roads).
The easier way is to have some idea where they are feeding, and then watch to see from which direction the birds arrive.
Knowledge of flightlines is essential for successful decoying as they will always determine which part of the field you set up in.