An Evaluation of the Method of Estimating Body Fat in Birds by Quantifying Visible Subcutaneous Fat

--Field ornithologists frequently quantify the fat reserves of smallto mediumsized birds by assigning a numerical rank to visible subcutaneous fat. Despite the significance and widespread application of this method, relatively few studies have tested its underlying assumptions. Using wintering Dark-eyed Juncos, Junco hyemalis, the visible fat-class method was evaluated in four ways. (1) The relationship between visible fat class and lipid index (angular transformation of g lipid/g lean dry mass) contained a significant linear term; nonlinear terms were nonsignificant. At least in statistical terms, these fat-class data closely approach interval data. (2) The Y-intercept of the lipid index-fat class relationship was significantly different from zero, indicating the presence of unseen lipid reserves at 0 fat class; the method therefore has limited utility in studies of birds involving severe food limitation. (3) Wintering juncos in southwest Indiana lost 43% of their visible fat overnight, suggesting that fat class methodology can measure short-term changes in body fat of small birds. (4) Important sources of error include inconsistent positioning of the bird during classification of visible fat, mistaking nonfat tissue (e.g., gut) for fat, difficulties with classification of fat in dead specimens, and observer bias. UNA EVALUACI•)N DEL M•,TODO PARA ESTIMAR LA CANTIDAD DE GRASA EN AVES, MEDIANTE LA CUANTIFICACI(•N DE LA GRASA SUBCUTANEA VISIBLE Sinopsis.--Los ornit61ogos frecuentemente cuantifican la reserva de grasa de aves, de pequefio a mediano tamafio, asignando un rango num•rico a la cantidad de grasa subcutfinea visible. A pesar de lo significativo y la amplia aplicaci6n de este m•todo, muy pocos estudios han puesto a prueba sus postulados. Utilizando individuos invernales de Junco hyemalis, evaluamos el m&odo de la cantidad de grasa subcutfinea en cuatro formas. (1) La relaci6n entre el rango asignado a la cantidad de grasa observada y un indice de grasa (transformaci6n angular de g lipidos/g de masa seca limpia) conteniendo t6rminos lineares ignificativos; no resultaron significativos los t•rminos no lineares. A1 menos en t6rminos estadisticos, los datos en rangos particulares de grasa, se acercan muy marcadamente a los datos de int•rvalos. (2) E1 intercepto en Y de la relaci6n del indice-grasa y rango de llpidos result6 significativamente diferente de cero, indicando la presencia de una reserva de grasa en el rango 0; este m•todo tienen una utilidad limitada en estudio de aves que envuelven una marcada carestla de alimentos. (3) Las aves estudiadas en el sur de Indiana, perdieron el 43% de la grasa visible durante la noche, lo que sugiere que el m•todo de asignar angos tan s61o puede medir cambios a corto alcance, en aves pequefias. (4) Otras fuentes importantes de error incluye el colocar en el rango equivocado a un ave, confundir tejido no-adiposo con grasa, dificultad en la clasificaci6n de grasa en animales muertos y sesgo por parte del observador. Fat reserves play important roles in the wintering, migratory and breeding phases of the annual physiological cycles of birds (Blem 1990). Ornithologists working in the field and laboratory commonly estimate • Current address: Department of Zoology, 6270 University Boulevard, University of British Columbia, Vancouver, British Columbia V6T 2A9, Canada.