How do ducks represents isolation for Holden apart from his thinking about them while talking to Mr. Spencer?
Holden's obsession with the ducks in Central Park, especially where they go in the winter, he is very curious to discover the habits of the ducks and how they disappear and then reappear when the weather gets warm, acts as a way for him to focus on questions that are difficult to answer, like why young people die.
Holden asks this question over and over again, failing to get a satisfactory answer. To me, this question about the disappearing ducks was always his way of asking about the isolation and transformation of death. Since, in my view, he is stuck in a cycle of grief over the loss of his little brother, he can't make sense of his death and cannot find his way back to participation in society without getting some answers.
Holden longs to understand the cycle that the ducks follow, he somehow equates it with a sense of eternity, where life is renewed each and every spring. It is comforting to think that there is life after death, especially thinking about Allie who will forever be a child.
Holden's desire to see the ducks again would provide him with comfort, they are familiar, they remind him of when he was a child. So in a sense even though Holden sits by the frozen pond and contemplates his own death, the ducks really provide something for Holden to live for, waiting for them to return gives him a sense of purpose.
Even though, he can't get a satisfactory answer to where the ducks go in the winter and how they know to come back, in his isolation of never receiving an answer, he is comforted by the question, a mystery of nature, one that provides clarity to other mysteries that cannot be explained, like death.
Twice Holden makes inquiries about where the ducks in Central Park's lagoon go in the winter (Chapter 9 and Chapter 12). Both times Holden asks cab drivers where they think the ducks go in the winter; his query is met with brusque replies, expressing astonishment and even anger that he would ask such an outrageous question. It seems that only Holden is concerned with the whereabouts of the ducks.
Then in Chapter 20 after Holden gets drunk and breaks the record he bought for Phoebe, he takes a walk in Central Park and looks for the ducks in the lagoon. He cannot, however, find any ducks, so once again he is alone, isolated from everyone.