Thoreau notices that the ice on the pond is pretty sensitive to shifts in temperature.
Speaking of ice and temperature, Thoreau has a story about that. There was an old local who decided to go duck hunting at Walden Pond. This guy decided to hide and wait for ducks, and he thought he heard the sound of an enormous flock landing on the pond. When he looked, it was just the melting ice plates rubbing up against the shore. Fooled!
Thoreau enjoys the sight of melting sand and clay, which turn into funny shapes like branches when they melt. It's almost as if the earth is expressing itself with its own poetry. Thoreau wonders if like melting sand and clay, human beings could change, too, under a more benevolent (nicer) influence.
Thoreau describes spring as a sudden burst of animation (no Pokémon, sorry), brought in by birds, squirrels, frogs, and tortoises. It's a time when people suddenly feel, innocent, cleansed of all their sins.
On April 29, Thoreau observes a night-hawk dancing in the sky. This is probably a personification. As far as we know, birds don't dance.
Fast-forward to May, when more kinds of birds make their appearance, and the pine trees cover the earth with their pollen. Pretty pretty, we'd say.