Holden says that he could think about two things at the same time while talking to Mr. Spencer. He observes that
You don't have to think too hard when you talk to a teacher.
What Holden was thinking about was the ducks in the lagoon near Central Park South.
I was wondering where the ducks went when the lagoon got all icy and frozen over. I wondered if some guy came in a truck and took them away to a zoo or something. Or if they just flew away.
Holden continues to worry and wonder about the ducks in Central Park after he gets to Manhattan. In Chapter 9 he asks the cab driver:
"You know those ducks in that lagoon right near Central Park South? That little lake? By any chance, do you happen to know where they go, the ducks, when it gets all frozen over?"
In Chapter 12 he asks another cab driver the same questions. This one becomes angry.
"How the hell should I know?" he said. "How the hell should I know a stupid thing like that?"
The driver is angry because he doesn't know the answer to the question.
Holden's concern about the ducks in wintertime shows a kind and thoughtful side to his character which is in contrast to his rebellious and pessimistic side. This is one of the ways in which Salinger makes him seem like a real person. His interest in the ducks also seems to indicate a personal desire to enjoy the same kind of freedom as those wild creatures. Holden is a lot like Huckleberry Finn in that he resents being "civilized"; he hates the feeling of being pressured to fit into a mold determined by society. He doesn't appear to know what he wants, but he seems to know what he does not want. He sees examples all around him of what he does not want.
It is significant that in the first chapter Holden describes a typical advertisement for a private school like Pencey as follows:
And underneath the guy on the horse's picture, it always says: "Since 1888 we have been molding boys into splendid, clear-thinking young men." Strictly for the birds. They don't do any damn more molding at Pencey than they do at any other school.
Holden keeps thiking about the ducks because the ducks are free. He keeps flunking out of schools because he wants to be free himself.