Temporal Patterns in Cause-specific Mortality of Northern Bobwhite in Northern Missouri

Northern bobwhite populations have been declining throughout most of their range. Population declines imply that mortality consistently exceeds production and therefore a thorough understanding of the causes and temporal distribution of mortality is important. We present Heisey-Fuller estimates of monthly survival and cause-specific mortality rates of 1,001 radio-marked bobwhite on private lands in northern Missouri during 1989-1992. Monthly survival rate was lowest during the hunting season (November-January)(jc = 0.639), intermediate during the breeding season (May-September) (x = 0.815) and highest during the remainder of the year (October, March-April)(jc = 0.858). Females had significantly lower (P = 0.027) survival rate (0.552) than males (0.653) during the first month of the hunting season. Males had lower (P = 0.06) survival rate than females during July (0.710 vs. 0.887). Avian and mammalian predators were primary causes of natural mortality. Avian mortality was highest during December, January, and February, then declined through May. However, avian mortality increased during the breeding season for male bobwhite. During June, males experienced greater (P = 0.012) avian mortality (0.073) than did females (0.025). This corresponds to the peak in whistling activity in Missouri and may represent a cost of "advertising" by males. Mammalian-caused mortality of bobwhites increased from March through July, reaching a maximum during the peak reproductive period and is associated with increased vulnerability of nesting and brood-rearing birds to olfactory predators.

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