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Early mortality and the numbers of blue grouse. Zwickel, Fred C.


This study was designed to test the hypothesis that the level of first summer mortality in blue grouse is determined by the condition of the hen and that this in turn determines the level of subsequent fall and spring densities. Early mortality was studied in a series of field and aviary comparisons of chicks on, or from, two areas of Vancouver Island that were in different stages of vegetative succession, following logging and burning. The summer recruitment each year was then compared to annual trends and mortality rates as determined for the breeding populations. No relevant differences were found in the pre-hatch parameters of recruitment (clutch size, fertility, and hatchability) between areas or years. No differences were found in the survival of young between areas, but differences were found between years. There were always sufficient young produced into the fall period to replace the annual losses in the breeding population. The major conclusions are: (1) early mortality does vary between years but does not vary between different habitat types or between areas with different breeding densities, (2) variations in early mortality between years are a result of as yet undetermined parental influences, and (3) this mortality is involved in the regulation of fall numbers, but is not involved in the regulation of subsequent breeding levels in established populations.

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