This theory is appealing because it explains not only the source of variation but also the benefits females receive for their choice of a mate. Unlike boat-tailed grackles, house sparrows are monogamous, and males aid females in rearing offspring. Thus, females could benefit from selecting males in good condition, because these males could be better able to help rear offspring. Also, if there is a genetic component of condition, males in good condition could pass on good genes to their offspring. Bib size in house sparrows is determined by the amount of melanin deposited in the feathers during the annual molt.
Melanin is a derivative of the essential amino acid phenylalanine.
Thus, bib size in house sparrows could reflect male condition if a male's foraging ability influences his intake of phenylalanine. To test for dietary limitations on bib production, we placed birds on synthetic diets that provided different levels of amino acids. Following molt, there was no difference in the bib size of the treatment groups, suggesting that, at the levels tested, amino acids in the diet do not limit the amount of melanin deposited in bib feathers.