Why is Holden obsessed with ducks?

The ducks usually leave in the winter and come back in the spring. Holden is worried they are going to die if they remain in the cold. I think Holden's obsession has a lot to do with his fear of change and mortality. Recall the scene near the lagoon when Holden looks for the ducks. In this scene, he obsesses about his own death and then about Allie.