Female Starlings With Experimentally Impaired Pre-Laying Condition Produce Smaller But Vitamin A Richer Eggs

Egg production is a main reproductive investment for females and a powerful pathway of maternal effects in oviparous species. The number and size of eggs in a clutch directly determine the amount and viability of the offspring. Similarly, yolk's key components, such as antioxidants (e.g., carotenoids, vitamins A and E) and hormones (e.g., androgens), can substantially shape offspring development and phenotype. External and internal factors may influence the female's ability to maximize offspring fitness through these egg characteristics. To study how female pre-laying body condition affects these variables, we experimentally impaired the condition of wild female spotless starlings (Sturnus unicolor) by clipping two flight feathers per wing before egg laying, and studied the impact on clutch size, egg size and yolk composition (concentrations of androgens and antioxidants, and oxidative damage on lipids). We found that females with impaired condition laid the same number of eggs as control females, but these were smaller. Our treatment did not significantly affect yolk concentrations of androgens, carotenoids, or vitamin E. However, females with impaired condition laid eggs with higher concentrations of vitamin A, and a weak but not significant tendency to have more oxidized lipids as compared to eggs laid by control females. These findings show that females differentially adjust each egg's characteristics and components depending on their condition. Also, our results suggest that females in a poor condition may strategically increase the allocation of vitamin A to yolks as a potential way to compensate for the negative impact of overall egg size.