significantly lower proportion of females are recruited as workers, reducing the overall level of sociality expressed in that population, ranging from above 75% in some years to below 45% in others, in the same location. Sex ratio is also affected by photoperiod at the time of egg production, with eggs laid on or near the summer solstice being almost exclusively male, so the earlier the season starts, the more female-biased the early brood phase will be, as in Kansas, where there are no males produced at all in the first brood, and there are three broods annually. Nest-site fidelity Nest-site fidelity may be due to one of three reasons, or a combination: Philopatry is the tendency for adult bees to excavate a nest near their natal nest; in H. rubicundus, females returning from overwintering sites typically excavate burrows within 30 cm of the location of their natal nest. Returning to the vicinity of the natal nest is beneficial because the site must have been successful enough to produce adults for one year. This prevents H. rubicundus from taking the risk of settling in a poor location for brood rearing. Habitat learning describes the process through which females recognize characteristics of the nest from which she came and chooses to nest in similar conditions. Although this is different from philopatry in that she will not purposely choose to be near her previous nest and will make selections based on environmental factors, the nest the female chooses will often still be near her original nest. Social facilitation may influence the nesting location chosen by a female because the benefits of nesting close to other bees may outweigh the costs of finding a new location with a suitable substrate. Gregarious nesting Dense nesting tendencies of H. rubicundus are most likely due to the following three factors: There is a limited amount of suitable substrate in which the bees can build their nests, so they must build many nests packed tightly together without compromising the structural integrity of the nest. As mentioned earlier, philopatry is an important factor in maintaining an aggregation. The search for a new nesting site requires a lot of resources, so females will likely limit their dispersal and stay near their natal nest sites. Hymenopteran and dipteran species may attack the ground nest of H. rubicundus. Although it would seem that aggregation of nesting would increase the mortality due to