This is from a poultry science journal:
Birds, reptiles and insects (e.g. Hymenoptera) share fundamental similarities in their reproductive strategies due to the presence, in the female tract, of specialized sites in which spermatozoa may reside for prolonged periods upon a single mating. In avian species, two distinct storage sites are present, one located in the utero-vaginal junction and the other in the lower portion of the infundibulum. At both sites, spermatozoa are stored in sperm storage tubules (SSTs) which are discrete, generally non-branched invaginations of the luminal epithelium ...The SSTs located in the uterovaginal junction are considered as the main site of residence of spermatozoa upon their deposition in the lower portion of the vagina. The fertilizing potential of females along the reproductive season is at first dependent on their ability to store and maintain adequate populations of spermatozoa in their SST in order to repeatedly provide the site of fertilization with sufficient numbers of fit spermatozoa after each ovulation. As a consequence in poultry species, eggs from females with prolonged sperm storage potential have also a natural tendency to maintain optimal fertility rates for prolonged periods. This situation may be highly desirable to sustain high fertility in the case of partial failure of the males during the season.
To put it simply, they store it in "special little pockets" so it doesn't fall out. If nothing else, this should make you consider your hens with a bit more respect now!
