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Conditioning of Sandhill Cranes During Fall Migration

Introduction

During fall migration, midcontinent populations of 3 subspecies of sandhill crane (G. c. canadensis, G. c. rowani, and G. c. tabida) stop for extended periods on traditional staging areas in the northern plains region of North America before continuing southward to wintering grounds located primarily in Texas and New Mexico (Johnson and Stewart 1973, Lewis 1977, Tacha et al. 1984). Cranes that nest in central and arctic Canada, Alaska, and eastern Siberia spend most of September and October on staging areas in eastern Alberta, Saskatchewan, Manitoba, North Dakota, South Dakota, and northwestern Minnesota (Lewis 1977, Melvin and Temple 1983). In North Dakota, subspecies composition varies by site. Cranes staging in the westcentral region (McLean County) are primarily G. c. canadensis, whereas cranes in the central part (Pierce and Kidder counties) are primarily G. c. rowani (Johnson and Stewart 1973). The fall distribution of the midcontinent population of G. c. tabida is centered in northwestern Minnesota, and their distribution in North Dakota is limited primarily to a small population in Kidder County (Johnson and Stewart 1973). The subpopulations of sandhill cranes that stage in Kidder and McLean counties spend winter primarily along the Texas Gulf Coast (50,000-70,000 cranes, Tacha et al. 1984) and western Texas (450,000 cranes, Iverson et al. 1985), respectively.

The significance of fall staging areas in the northern plains region as conditioning sites for the midcontinent sandhill crane population is poorly understood. The prolonged stay on these staging areas probably prepares birds physiologically for the continuation of fall migration (Melvin and Temple 1983) and possibly for winter, much as spring staging areas along the Platte and North Platte rivers physiologically prepare cranes for spring migration and reproduction (Krapu et al. 1985). However, fat levels among migrant sandhill cranes collected in Oklahoma in October 1979 were much lower than observed late in the spring staging period of the same year in Nebraska (Iverson 1981). This difference suggests, at least during some years, that cranes acquire less fat during autumn staging intervals in the Great Plains region than during spring, possibly because of less suitable foraging conditions on fall staging areas.

We address (1) patterns of body mass change among adult G. c. canadensis and G. c. rowani on staging areas in North Dakota during the fall stopover and (2) the magnitude of change in body nutrient composition of G. c. rowani from early to late in the fall staging period.

The North Dakota Game and Fish Department and cooperators provided measurements on hunter-shot cranes; B. A. Hanson and R. O. Woodward assisted in collecting and processing cranes, and C. R. Luna prepared the figures. D. P. Fellows gave constructive comments on an earlier draft of the manuscript.

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