Parasitism of greater prairie-chicken nests by ring-necked pheasants

We studied nest parasitism of greater prairie-chickens (Tympanuchus cupido pinnatus) by ring-necked pheasants (Phasianus colchicus) as a possible contributing factor in the decline of an isolated population of prairie-chickens in Jasper County, Illinois. Both species nested in small, scattered grasslands maintained on prairie-chicken sanctuaries. Incidence of parasitic laying by pheasant hens in prairie-chicken nests increased from 2 to 43% between 1970 and 1983 and remained high through 1987. Nest success (≤1 host-egg hatching) did not differ (P = 0.33) between 60 unmanaged parasitized nests (43%) and 602 unparasitized nests (51%). However, success of 14 parasitized prairie-chicken nests managed by removal of pheasant eggs (86%) was greater (P = 0.02) than for 24 unmanaged parasitized nests (46%) during 1983 and 1985-87. Hatchability of fertile prairie-chicken eggs was less (P < 0.01) in parasitized nests (77%, conservatively) than in unparasitized nests (94%), because of earlier hatching of pheasant eggs, increased embryo morality of prairie-chickens, or increased nest abandonment. Large clutches of prairie-chicken eggs typical of early nests were more likely (P < 0.001) parasitized than small clutches laid later. Factors correlated with rate of nest parasitism included numbers of pheasant cocks (P = 0.01) and numbers of pheasant nests (P < 0.001) found each year. Although pheasant control apparently eliminated nest parasitism during 1988-94, prairie-chicken numbers continued to decline. Without management intervention to control pheasants on sanctuaries, the survival of this isolated, remnant flock of prairie-chickens may be in greater jeopardy.