Timing of reproduction and egg quality covary with temperature in the insectivorous barn swallow (Hirundo rustica)

1. Egg size and composition can be predicted to trade-off against current condition of reproducing females. Adverse conditions for reproduction should thus give rise to a reduction in egg size and quality. 2. Here we analyse timing of reproduction, egg mass and hatchability, and concentration of maternal carotenoids and components of the immune system in the eggs in relation to ambient temperature during the breeding season for the Barn Swallow (Hirundo rustica, Linnaeus). 3. Relatively high temperatures during one day enhanced the probability that clutches were initiated the following days. Egg mass at laying, concentration of lutein, the main carotenoid in Barn Swallows, and lysozyme, a major maternal component of innate immunity in eggs, but not maternal antigen-specific antibodies, covaried with temper-ature during the days preceding laying of individual eggs. 4. Egg mass but not laying order or temperature during incubation predicted hatch-ability of the eggs. 5. This study suggests that temperature affects not only egg mass, as previously shown for bird species, but also concentration of maternally derived substances, such as antioxidants and immune factors, that can affect egg hatchability and offspring antiparasite defence and viability.