Pollen

Pollen clings to the hair of a honeybee. The pollen is dry. The bees will regurgitate nectar from their honey stomach, then rake their legs over their body. The nectar will get mixed into the dry pollen. As the bees groom themselves, they rake the now moist pollen into a pellet and it is attached to their pollen baskets on their rear legs.

There is pollen coming in right now. Hives are limited on collecting pollen by cool temperatures and rain. If it is cool in the early part of the day, sometimes pollen collection is limited to just a few hours a day. Rainy days keeps the bees in the hive.
Strong colonies use quite a bit of pollen this time of year. Keeping pollen patties on through the month of May assures proper nutrition for the spring brood build up.
 By not providing the needed nutrition can hurt the overall quality of the bees. When bees don't get the proper pollen during their development, the resulting bee, can have a shorter life spans and their glands may not develop properly.
 I always keep a pollen patty on my hives. As the calendar moves into May, a half a pollen patty sits on the top bars of my hives. I check them weekly and replace as needed. The weather is always hard to predict but I want healthy bees as we move towards the June nectar flow.