We pick a nest that has just-hatched young (so that they can't wander), and place a cage over it that has a trap door. The door closes on the tern when it goes to the nest to feed its young. We do one round to try to catch the first parent, then another right after to attempt to get the other. To be able to track a breeding couple is very helpful, especially if they breed on the island each year.
Handling the larger adults does require a bit more muscle and finesse than the chicks, but somehow they still manage to look elegant even when they're not cooperating.
They are then taken to headquarters for processing.
Note that the first one below has some white speckling above its bill; that tells us that it is not yet a fully mature adult and is likely two to three years old. More on processing later...