The amount of pollen brought back to a hive seems to depend on two things: the presence of brood in the hive and a genetic predisposition to hoard pollen. The relationship between these two phenomena was recently studied by Jennifer Tsuruda and Robert Page, Jr. and reported in the journal Behavioral Ecology & Sociobiology.
They selected two strains of bees for their study—those that hoarded large quantities of pollen and those that collected little pollen. Then they exposed part of each group to large amounts of brood.
They found that the bees that hoarded pollen brought in even more pollen when brood was present, but the low pollen collectors didn’t change their collection habits when exposed to the brood. The authors concluded that both factors play a role in pollen collection behavior, but the amount of influence can vary between the different strains of bees.
Other studies have shown that, as a general rule, pollen is collected by bees on an as-need basis. Colonies usually store only a few days-worth of pollen, yet they store vast quantities of nectar in the form of honey.
Pernal and Currie (2002) found that colonies reacted to low pollen stores by simply sending out more foragers, not by increasing foraging rates. They also found that the bees responded to low quality pollen in the same way—by bringing in a greater total amount of pollen. In other words, they did not change the type of pollen collected when it was low quality, they just collected more of it.
Since pollen is collected externally (in pollen baskets or on the body), the bees are unable to discern the quality of pollen in the same way as they do with nectar, which they taste. This inability to distinguish pollen quality is one of the major reasons that bees must have a wide selection of floral resources in order to maintain good health.
 Tsuruda, J. M. and R. Page, Jr. 2009. The effects of young brood on the foraging behavior of two strains of honey bees (Apis mellifera). Behavioral Ecology and Sociobiology 64: 161-167.
 Pernal, J. H. and R. W. Currie. 2001. The influence of pollen quality on foraging behavior in honeybees (Apis mellifera L.). Behavioral Ecology and Sociobiology 51: 53-68.