Effects of environmental conditions on the egg mass, yolk antioxidant level, eggshell thickness and eggshell spotting patterns of Great Tits (Parus major)

Avian mothers can influence the fitness of their offspring by resource investment into the egg. Allocation of macro- and micronutrients into the eggs may be costly for the female, therefore, we expect that resource invest- ment may be affected by the environmental and social conditions the mother experiences during egg formation. Here, we investigated whether environmental circum- stances experienced by the reproducing female exert an influence on egg mass, yolk antioxidant (lutein and toco- pherol) concentration, eggshell thickness, and the eggshell spotting patterns of Great Tits ( Parus major ). Our study showed that when caterpillars were less abundant, in colder weather and when breeding in an area of higher local breeding density, female Great Tits laid eggs of lower mass, suggesting that adverse environmental circumstances constrain the macronutrient investment into the eggs. However, we found no evidence that yolk antioxidant concentration and eggshell thickness were affected by environmental factors. Female Great Tits may use their endogenous stores or spend more time and energy in finding sufficient amounts of dietary antioxidants and cal- cium under unfavourable environmental conditions, which may have a cost effect on their own conditions. We found that birds that bred in the beginning of the season laid eggs with darker eggshell spots. Moreover, females laid more spotted eggs in colder weather and when breeding in higher density areas. Our results suggest that Great Tits deposit more of the potentially harmful pro-oxidant protoporphyrin pigment into the eggshell under unfavourable environ- mental conditions