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14 <br />varents separate trom their young of the previous year upon departure from Aransas NWR in <br />northward migration, while on route to the breeding grounds or soon after arrival on the <br />breeding grounds (Allen 1952, Archibald et al. 1976, Stehn unpubl. 1992, Kuyt unpubl.). <br />Information on marked individuals suggests that most juveniles and subadults spend the <br />summer near their natal area (Kuyt 1979b, 1981a). <br />E. Reasons for Listing <br />The impact of human settlement upon the wildlife of interior North America is dramatically <br />evident in the changing status of the whooping crane. Cranes disappeared as agriculture <br />claimed the northern Great Plains of the U.S. and Canada (Allen 1952). Only one small <br />population survived. Ironically, the traditions which appear to have saved the whooping <br />crane as a small relict breeding population in WBNP, prevent its voluntary return to what <br />was once its principal nesting range. Re- colonization of these former breeding areas <br />remains unlikely unless man assists with purposeful reintroduction. <br />Biological Characteristics: Delayed sexual maturity, small clutch size, and low recruitment <br />rate preclude rapid population recovery. The current northern breeding grounds may be <br />another handicap to productivity because the ice -free season is only 4 months. During that <br />time, pairs must incubate their eggs for 29 -31 days, and rear their chicks to flight age in 3 <br />months. Consequently, unless nest loss occurs early in incubation, there is rarely time to <br />lay a second clutch and fledge young if the first clutch fails. During 1939 -1964 when there <br />was no human interference in the form of egg removal, 180 breeding pairs produced 15 sets <br />of siblings or one of each 12 families arriving on the Texas coast in fall contained 2 juveniles <br />(pers. comm. E. Kuyt 1993). <br />During years when whooping cranes were surveyed on the breeding grounds (when no eggs <br />have been removed), about one out of every four hatched chicks survived to reach the <br />Texas coast. Factors which limit chick survival remain open to conjecture. Most mortality <br />occurs soon after hatching, and chicks that fledge have a high probability of successfully <br />completing their first migration (Kuyt 1976a). Most immediate post- hatching mortality may <br />be related to sibling aggression and short-term food shortage because eggs hatch <br />asynchronously and the precocial young are extremely aggressive toward each other. The <br />dominant chick apparently obtains principal access to food made available by the parents, <br />consequently brood -size is rapidly reduced during periods of food shortage (Miller 1973, <br />Drewien 1973). Prolonged food shortage, possibly related to drought, and <br />drought- increased predation (Kuyt 1981.b) may account for additional mortality. Suitable <br />nesting habitat conditions are the chief reasons for population increases 1984 through <br />1990. <br />Little is known about the impdrtance of diseases or parasites as mortality factors. At the <br />time of his capture (mid- September) in WBNP, due to a wing injury (Novakowski 1965), <br />"CAN -US" was found to be infected with coccidia. Coccidia have been found in whooping <br />crane droppings collected on the wintering grounds (Forrester et al. 1978), and have caused <br />deaths of several whooping crane chicks at Patuxent Wildlife Research Center (Carpenter et <br />al. 1980). Fecal accumulations and concentrations of coccidia oocysts at brooding sites on <br />the breeding grounds may infect preflight birds. Chicks may be especially vulnerable to <br />attack by coccidia due to the absence of acquired immunity. However, droppings normally <br />