The Effect of Dietary Lysine Level on the Energy and Nitrogen Balance of the Dark-Eyed Junco

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Abstract
Dark-eyed juncos [Junco hyemalis] were fed 8.5% protein wheat diets supplemented with amino acids. The control diet was supplemented with lysine, tyrosine, threonine, leucine, isoleucine and valine. The low-lysine diet was similarly supplemented except that 0.2% urea was substituted for 0.2% lysine. Neither group of birds showed significant changes in either body weight or body fat depots. Food consumption and excrement production were not significantly different between the 2 groups of birds. The birds fed the control diet had a significantly higher efficiency of energy utilization, although there was no significant difference in the metabolized energy between the 2 groups. The birds fed the control diet (0.4% lysine) had significantly higher metabolized N utilization than those fed the low-lysine diet (0.2%) lysine.) The metabolized N and efficiency of N utilization were both negative for the low-lysine birds. The inability of the low-lysine birds to maintain N balance demonstrates the essentiality of lysine for dark-eyed juncos. Wintering juncos, as well as other small passeriforms, may maintain proper nutritional balance by eating a variety of seeds, each of which by itself may be deficient in lysine and/or other amino acids, but which, in combination provide adequate amino acid and energy balance for subsistence. The 8.5% protein control diet with a combustible energy content of 3871 cal/g (calorie:protein ratio (C/P) = 455) was considered adequate for laboratory maintenance of juncos, since it permitted maintenance of body weight, high efficiencies of energy and N utilization, and maintenance of positive N balance. Diets with 8-9% protein and a C/P of 450-500 appear to be best as laboratory diets for juncos and probably other small passerine birds. The possibility of deleterious effects of 15-16% protein poultry diets with a C/P near 250 are discussed.