Organ mass dynamics in relation to yolk precursor production and egg formation in European starlings Sturnus vulgaris

Egg production in passerines and other birds requires rapid synthesis of pr oteins and lipids. We hypothesized that these biosynthetic demands would ne cessitate hypertrophy of the liver, which produces the yolk precursors vite llogenin and very low-density Lipoprotein (VLDL), and of the metabolic mach inery that supports the liver's biosynthetic activity (e.g., heart, kidneys , lungs, and digestive organs). To test this hypothesis, free-living female European starlings (Sturnus vulgaris) were collected through two breeding seasons. Change in liver mass in relation to breeding stage differed betwee n years, as did the relationship between liver mass and plasma vitellogenin levels. In the first year, dry lean glycogen-free liver mass showed little seasonal variation and was not correlated with vitellogenin levels among e gg-laying females. In the second year, liver mass was 4%-44% greater during egg laying than at other stages of breeding and was positively related to vitellogenin levels. In both years, the mass of the liver was not related t o plasma VLDL levels. Thus, we did not find consistent relationships betwee n liver mass and its biosynthetic output. In contrast to our hypotheses, th e masses of the heart and digestive organs were lower during egg laying tha n they were before breeding. Meeting the biosynthetic demands of egg produc tion does not appear to require hypertrophy of the liver or supporting meta bolic machinery.