Journal of Field Ornithology (Jun 2022)
Fat scoring in four sparrow species as an estimation of body condition: a validation study
Abstract
In avian biology, body condition has been positively correlated with survivability, reproduction, migration, and habitat quality. A common method of assessing avian body condition, which typically refers to measures of energy stores, is a qualitative scale based on visible subcutaneous fat. However, the only accurate way to determine the lipid composition of a bird is to chemically extract it, which is fatal to the bird and time-consuming for researchers. The goal of this study was to determine the relationship between fat score and percent body fat measured by lipid extraction in two sparrow genera: 41 White-throated Sparrows (Zonotrichia albicollis), and 19 Song Sparrows (Melospiza melodia), 5 Swamp Sparrows (Melospiza georgiana), and 3 Lincoln’s Sparrows (Melospiza lincolnii). All birds died from building collisions. All birds measured within the standard weight range for their species and scored within the 0.5–4 range using the European Science Foundation (ESF) system from the British Trust for Ornithology, which is typical for wild birds. Each bird was dissected ventrally to remove fat pads, the remaining carcass was blended with distilled water, and then the fat pads and slurries were dried separately at 100°C for 24 hours. Crude fat content of fat pads and the homogenized carcass was measured through high-temperature solvent extraction. Fat score and percent body fat was significantly positively correlated for all species, and linear across all sparrows. A unit increase in fat score equaled a mean increase of 0.52±0.06g or 2.1±0.3% of percent body fat. However, the variance in percent body fat increased with fat score, suggesting that the relationship may become less reliable in fat scores above 4. We conclude that fat scoring offers a quick method of assessing body condition in small wild passerines but may not provide accurate lipid content, especially for fatter birds.
