Evidence of a Decline in Fat Storage by Midcontinental Sandhill Cranes in Nebraska During Spring: A Preliminary Assessment

Authors

Gary L. Krapu, USGS Northern Prairie Wildlife Research CenterFollow
David A. Brandt, USGS Northern Prairie Wildlife Research CenterFollow
Deborah A. Buhl, USGS Northern Prairie Wildlife Research CenterFollow
Gary Lingle, Platte River Whooping Crane Maintenance Trust, Wood River, NE

Date of this Version

2006

Comments

Published in PROCEEDINGS NORTH AMERICAN CRANE WORKSHOP 9:179-184.

Abstract

When an ice storm killed an estimated 2,000 sandhill cranes (Grus canadensis) in the Central Platte River Valley (CPRV) in Nebraska on 24 March 1996, we retrieved the fresh carcasses of 118 adults to test for a decline in the condition of spring-staging cranes from that date in 1978 and 1979. We first conducted a principle component analysis on 3 morphological variables (tarsus, exposed culmen, and wing chord [flattened]) and used the first principal component (PC1) as an index of body size. Then, to account for variation in body mass due to size, we regressed body mass on PC1 and computed an adjusted body mass using the residuals from this regression. For a sample of lesser sandhill cranes (G. c. canadensis) (n = 147) collected in 1978-79 and 1996, PC1 explained 63% of the total variation in body size and for greater sandhill cranes (G. c. tabida) (n = 75), 72% of variation. For both subspecies in 1978-79, the relationship between adjusted body mass and date depended on sex (P < 0.08) but not year (P > 0.8). For 24 March (the date of the ice storm in 1996), no significant differences in adjusted body mass were detected between 1978 and 1979 or between males and females for either subspecies. For lessers, the mean adjusted body masses for 1996 were 69-152 g less than the mean adjusted body masses predicted for 24 March 1978 and 1979; significant differences were detected only for males between 1996 and 1979 (difference = -152.5, SE = 75.9). For greaters, mean adjusted body masses for 24 March 1996 were 175- 415 g less than the mean adjusted body masses predicted for 24 March 1978 and 1979; significant differences were detected only for females between 1996 and 1979 (difference = -415.1, SE = 126.5). In 1978 and 1979, adjusted body mass was correlated with total body fat. These results are not conclusive but do support the hypothesis that at least part of the sandhill crane population was carrying less fat on 24 March 1996 than on the same date in 1978 and 1979. Further studies are needed to gain greater insight into how fat storage patterns have been affected by declining waste corn in the CPRV and significance of this change to the midcontinental sandhill crane population.