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Conditioning of Sandhill Cranes During Fall Migration

Results


Body Mass

Body masses of adult male G. c. canadensis were 3,406 52 (SE) g during 1-10 September and increased to 4,087 89 g by 21-30 October (Fig. 1). Adult male G. c. rowani mass increased from 4,169 27 g to 5,005 111 g during the same period (Fig. 1). Adult females exhibited similar patterns in mass gains, with female G. c. canadensis changing from 3,074 41 g during 1-10 September to 3,624 34 g by 21-30 October, whereas adult female G. c. rowani increased from 3,735 26 g to 4,351 121 g during the fall. Mass gains of G. c. canadensis from early September to late October averaged 20% for adult males and 18% for adult females, whereas mass gains of G. c. rowani averaged 20% for adult males and 17% for adult females. Rates of mass gain differed by sex (ANOVA, P < 0.002), but not by subspecies (P > 0.10). Body mass of juvenile G. c. rowani collected for carcass composition analyses increased 20% from 3,380 459 g (n = 4) in early September to 4,053 277 g (n = 11) in mid-October.

Figure 1: Line graph showing body mass curves for G.c. rowani (male and female) and G.c. canadensis (male and female) for September and October
Figure 1.  Body mass curves of adult male and female G. c. rowani and G. c. canadensis by 10-day intervals during fall in central North Dakota. Dashed lines represent time intervals where sample size was ≤5 cranes.

Above and beyond the variation associated with subspecies, sex, and time period, mass of adult cranes also varied among years (ANOVA, P < 0.0001). Much of the annual variation was caused by higher mass in the early years of the study (Fig. 2). Mass tended to decrease beginning in 1977, when the hunting season in North Dakota shifted from November to September. Even during the 1977-87 period, some annual variation in mass of cranes was evident, suggesting the influence of other environmental factors.

Figure 2: Line graph showing population marginal means of body mass of sandhill cranes
Figure 2.  Population marginal means (least-squares means) of body mass of sandhill cranes in central North Dakota, 1970-87. Population marginal means are the averages expected if there had been equal numbers of cranes in all categories of subspecies, sex, and time period.

Carcass Composition

Adult and juvenile G. c. rowani arriving in central North Dakota in late August and early September were lean (Fig. 3). Percentages of fat, protein, and ash did not differ between the sexes among adults (P > 0.10). Mean fat levels among adults increased from 177 11 g to 677 59 g. Fat levels of juveniles increased from 83 23 g in late August through mid-September to 482 32 g in mid-October. Fat content as a percentage of total body mass increased from 4 to 14% among adults and from 2 to 12% among juveniles. The estimated daily rate of fat gain for G. c. rowani was 10.1 1.4 g (n = 20) among adults and 8.0 1.0 g (n = 14) among juveniles during the fall staging interval. Fat-free mass of adults remained constant (978 vs. 981 g), whereas juveniles increased by 12% (from 740 to 829 g).

Figure 3: Bar graph showing the early and late body composition (fat, protein, water, ash, other) of juvenile and adult sandhill cranes
Figure 3.  Body composition of juvenile and adult sandhill cranes collected during early fall (21 Aug-12 Sep) and late fall (16-18 Oct) in central North Dakota.

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