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32 <br />Table 4. Age of whooping cranes when they first produced eggs, PWRC, 1975 - 1993'. <br />Age Producing Percent <br />(years) Femalesb Females` Producing <br />5 <br />28 5 <br />17 <br />6 <br />22 <br />8 <br />36 <br />7 <br />15 <br />8 <br />53 <br />8 <br />13 <br />10 <br />76 <br />9 <br />10 <br />9 <br />90 <br />10 <br />9 <br />9 <br />10b <br />a Does not include birds transferred between centers before maturation. Transfers <br />delayed egg production. <br />b Females reaching or passing through that age class by 1993. <br />c Females producing eggs in that age class. <br />eggs have been incubated under sandhill cranes or whooping cranes. Since these <br />modifications were undertaken, hatchability and chick survival has equaled that observed in <br />eggs and chicks from the AWP (Table 2). <br />Between 17 September and 4 November, 1984, seven (two male, five female) whooping <br />cranes in the captive flock at PWRC died from EEE. These losses represent a serious <br />setback to the captive breeding program because five of the birds were females. The <br />January 1985 sex ratio in the surviving adult captive population was 10 males to 4 females. <br />Sandhill cranes at PWRC also were exposed to the virus, but no mortality occurred <br />(Carpenter et al. 1987). Whooping cranes appear especially susceptible to EEE, <br />consequently the potential impact of this disease will be considered when selecting any site <br />for additional whooping crane populations. <br />Thirteen of the 32 whooping cranes at PWRC were exposed naturally to the virus and all <br />developed antibody titers. Birds that survive an EEE infection become immune, thus, the <br />use of an EEE vaccine should reduce the risk of this disease in the future. Actions taken in <br />1985 and continued annually to prevent another outbreak of EEE at PWRC included: (1) a <br />surveillance and control program for the principal mosquito_(C I� iseta melanura), vector of <br />the disease; (2) testing EEE vaccines and developing a more effective vaccine for whooping <br />cranes; and (3) continuing serological monitoring of the captive flock for antibody titers. <br />Now that the etiology of the whooping crane deaths at PWRC is known, it is expected the <br />disease threat can be minimized at PWRC by initiating appropriate mosquito control <br />measures and the use of EEE vaccines. However, the long -term efficacy of the vaccine is <br />unknown and annual boosters shots are required. The Crane Health Advisory Team <br />recommended that the cranes at ICF and PWRC be vaccinated but not the cranes at Calgary <br />and San Antonio Zoo. <br />In September - October 1987 a mycotoxin in the commercially prepared crane feed poisoned <br />about 240 of the 300 captive cranes at PWRC. Fifteen cranes died (5 percent of the flock), <br />