Embryo development remains suspended until the onset of incubation. This may be adjusted by incubation time and dilution. Experiments generally start after 24 hours of incubation. Technology incubators account for 39% of incubation programs. Thus the birthing process consists of laying eggs and external incubation. The duration of egg incubation is a matter of local preference. Fertile chicken eggs hatch at the end of the incubation period, about 21 days. Both sexes continue to add material to the nest during the incubation and nestling periods. The first egg hatches after 31 to 36 days of incubation. The average incubation period is 5 years. During daylight females can do as much as 96% of the observed incubation. They have a communal approach to the incubation of eggs and raising of young. Compare this to influenza which has 1-4 day incubation period. Both parents participate in the incubation of the eggs. Some of the adults that lose their clutch early in the incubation period renest.