Vitellogenin dynamics during egg-laying: daily variation, repeatability and relationship with egg size

Daily variation in circulating levels of the avian yolk precursor, vitellogenin (VTG), throughout the laying cycle was investigated in female zebra finches Taeniopygia guttata and compared with predicted ovarian follicle demand (based on a model of follicular development for this species). In general, the pattern of variation in plasma VTG matched the predicted demand from the developing ovarian follicle hierarchy. Plasma VTG was non-detectable in non-breeders, but increased rapidly with onset of yolk development, remaining high (1.43–1.82 μg/ml, zinc) through to the 3-egg stage. Plasma levels then declined at the 5-egg stage (to 0.78±0.32 μg/ml) and were undetectable at clutch completion. This result is consistent with the hypothesis that yolk precursor production is costly and that selection has matched supply and demand. While inter-individual variation in plasma VTG was marked (e.g. 0.47–4.26 μg/ml at the 1-egg stage), it also exhibited high intra-individual repeatability (r=0.87–0.93). Finally, we examined the relationship between plasma VTG and primary reproductive effort. While individual variation in plasma VTG was independent of clutch size, laying interval and laying rate, there was a complex, diet-dependent relationship between VTG and egg size, with low plasma VTG levels being associated with both very small (<0.90 g) and very large (>1.15 g) egg sizes.