Holden keeps asking them, if they knew where the ducks who live in Central Park South lagoon go when the water freezes over, which makes them (cab drivers) to question is mental state.
Explanation:
From the given question, Holden keeps asking the different can drivers if they knew where the ducks who live in Central Park South lagoon go when the water freezes over.
Also, when he asked same question to a taxi driver, the taxi-driver frowned at his question and then begins to talk about something else like fish instead. before now Holden asks the taxi-driver to have a drink with him, which he rejected.
