What do monarchs do at the overwintering sites?
While overwintering, monarchs don't do much! They rest on the tree branches, and occasionally fly around if the weather allows it. Monarchs need to consume enough milkweed as caterpillars and nectar as adults before reaching the overwintering sites so that they do not need to eat to survive while they are overwintering. There is not enough nectar available at the overwintering sites to sustain that many monarchs for the entire winter, but there is typically some nectaring behavior during the winter months.
Monarchs do need water during the winter, so on warm days they find streams or water from dew or fog/clouds on the mountain. They need moisture so that they don't become dehydrated.
They remain fairly inactive clustering in trees to conserve energy/lipid reserves, but do fly some on warm days and warm their wings in the sun. Towards the end of the season, monarchs may also begin mating in Mexico as they prepare for the journey northward.
Predation by birds also happens at overwintering sites, but monarchs cannot do much to avoid this.