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 proteins and lipids. We hypothesized that these biosynthetic demands would necessitate hypertrophy of the liver, which produces the yolk precursors vitellogenin and very low-density Lipoprotein (VLDL), and of the metabolic machinery 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 between 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 egg-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 to plasma VLDL levels. Thus, we did not find consistent relationships between liver mass and its biosynthetic output. In contrast to our hypotheses, the masses of the heart and digestive organs were lower during egg laying than they were before breeding. Meeting the biosynthetic demands of egg production does not appear to require hypertrophy of the liver or supporting metabolic machinery.