Date of Award:

5-1-1999

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Committee Chair(s)

James A. Gessaman

Committee

James A. Gessaman

Committee

James D. Fry

Committee

Kimberly A. Sullivan

Abstract

I evaluated the use of three direct methods (fat scoring, condition indices, and multiple regression of external morphological variables) and one indirect method to estimate total body fat (TBF) in Sharp-shinned Hawks (Accipiter striatus) and Cooper's Hawks (A. cooperii). All three direct methods explained a reasonable amount of variation in actual TBF values (r2 ≥ 0.82), and all three methods required the use of four equations to account for the categories of species, age, and sex. Indirect methods are those that predict lean mass and subtract the lean mass estimate from the actual body mass to produce an estimate of TBF. I estimated lean mass from external morphology, and this technique was also fairly accurate (r = 0.94). The methods evaluated here, though reasonably precise, may not be accurate enough to reliably compare estimated TBF between individual birds. Using multiple regression to directly estimate TBF from mass and tarsus length measurements is the recommended technique because it provided continuous estimates of TBF and the independent variables are less subjective than fat scores. I also studied the energy reserves of migrating Sharp-shinned and Cooper's Hawks captured as they traveled south along the Manzano Mountains in central New Mexico during the fall, 1992-6. I compared species, age, and sex cohorts for differences in mean percent TBF and found that (1) adults had greater stores than juveniles (6.1% vs. 4.3%), and (2) females stored more fat than males (6.0% vs. 4.4%). The significant age and sex effects indicated that larger age and sex groups (adults and females) carried relatively greater fat reserves than smaller age and sex groups (juveniles and males). Though female Cooper's Hawks were the fattest group, male Cooper's Hawks carried less fat than male Sharp-shinned Hawks. These results are thus not consistent with (1) the idea that body mass-related differences in energy expenditure constrain fat accumulation in these birds and (2) patterns expected for birds minimizing their predation risk. Age-specific differences in mean TBF were strong despite minimal differences in mass. This pattern probably reflected differences in hunting efficiencies and the extra costs immature birds face in reaching adult lean masses. Barring additional food acquisition, most birds carried enough fat reserves to survive from one to three days. Not surprisingly, adults showed lower starvation risk than juveniles, which is consistent with the high mortality rates for juvenile birds. Seasonal trends in fat reserves were apparent only for female Sharp-shinned Hawks, and the subtlety and scatter of the trends make this finding difficult to interpret.

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