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29 abnormalities, and leg disorders resulting from rapid growth (Kepler 1978). Carpenter and Derrickson (1981) summarized all mortalities in the captive flock from 1967 to 1981. Eggs were first produced at PWRC in 1975, when one female laid three eggs (Table 3). Although two females produced eggs when they were 5 years old, most captive females have not laid until they were 7 -11 years old (Table 4). Factors identified as responsible for delaying reproduction in the captive flock include rearing conditions, dominance relationships, age of separation of potential pairs from bachelor flock, sexual compatibility, inadequate pen size, and stress associated with handling and disturbance (Kepler 1976, 1978, Derrickson and Carpenter 1981). Between 1975 and 1993, the captive flock at PWRC produced 356 eggs (Table 3). Seventy -three whooping crane eggs were transferred from PWRC to Grays Lake between 1976 and 1984. To date, annual production has been primarily limited by the number of breeding pairs, and egg fertility. Although productive pairs at PWRC exhibit copulatory behavior, and males regularly attempt to mount their mates, successful natural copulations were not observed until 1991. In the spring of 1991, a pair of full- winged, behavioral conditioned, captive- reared whooping cranes, laid the first fertile egg at PWRC without artificial insemination. The Service believes naturally fertile pairs will lay more eggs than artificially inseminated birds. Natural fertilization reduces the risk of injury due to handling. To avoid imprinting problems, PWRC now rears chicks outdoors with a pair of whooping cranes or hand -rears them in visual and auditory contact with a subadult whooping crane role model. In 1992, five additional pairs produced five chicks by natural breeding. To acquire fertile eggs from badly imprinted or handicapped individuals, the females have been artificially inseminated using a variation of the massage technique (Gee and Temple 1978). In order to condition pairs to this procedure, the collection of semen from males and the handling of females are initiated well in advance of laying. Females are inseminated from the time their pubic bones begin to separate until laying ceases. Throughout this period, females are routinely inseminated three times per week and after each oviposition. From 1975 through 1981, 55 of 61 eggs (90 percent) were fertile, from 1982 through 1986, 89 of 97 eggs (92 percent), and from 1987 through 1992, 41 of 53 eggs (77 percent) were fertile from whooping cranes artificially inseminated. Fertility of artificially inseminated eggs over the entire period through 1992 averaged 86 percent. Between 1987 and 1992, the PWRC flock produced 74 eggs (50 fertile). From these 50 and 43 other PWRC eggs obtained from WBNP, PWRC fledged 49 birds. During the same interval, WRC shipped 22 birds to ICF in 1989 to help establish a second captive flock and 6 birds to Florida for release in 1993. The flock was split to reduce the risk of disease outbreaks decimating the entire captive population. Early attempts to artificially incubate whooping crane eggs suggested problems with less than optimum incubation regimes. Results obtained in 1978 supported this conclusion, because (1) hatchability of the 8 eggs retained at PWRC and incubated under sandhill cranes was 88 percent, (2) of 11 fertile eggs which were incubated artificially before their transfer to GL, only 5 hatched; and (3) the 6 fertile eggs that did not hatch at GL contained 4 late dead, and 2 early dead embryos. Hatchability of whooping crane eggs incubated by cranes exceeds that of eggs incubated in incubators. As a result, since 1979 all whooping crane