In many small hibernating species, food caching can replace fattening before winter.
Food caching means collecting and storing edible material for later use. Instead of converting a large autumn food supply into body fat, some small animals build hidden reserves in burrows, nests or scattered locations. They can then draw on those supplies during winter activity or during brief arousals from torpor.
This strategy is especially useful for animals whose small bodies cannot carry enough stored fat to last through an entire winter. Caching also lets an animal remain relatively lighter and more mobile before winter, although stored food must be protected from theft, spoilage and competing animals.
The alternative answers describe behaviours that may occur in particular species but do not replace pre-winter fattening in the general hibernation strategy. Nest building can provide insulation, while migration changes location; neither is an energy reserve. Sun basking is not a standard substitute for storing fuel.
Not every hibernator relies on caches. Larger animals commonly become hyperphagic—eating heavily and depositing energy as fat—whereas many small species depend on stored seeds, nuts or other foods.