Dietary carotenoid supplementation facilitates egg laying in a wild passerine

During egg laying, females face a trade-off between self-maintenance and
investment into current reproduction, since providing eggs with resources
is energetically demanding, in particular if females lay one egg per day.
However, the costs of egg laying not only relate to energetic
requirements, but also depend on the availability of specific resources
that are vital for egg production and embryonic development. One of these
compounds are carotenoids, pigments with immuno-stimulatory properties,
which are crucial during embryonic development. In this study, we explore
how carotenoid availability alleviates this trade-off and facilitates egg
laying in a small bird species, the blue tit (Cyanistes caeruleus). Blue
tits have among the largest clutch size of all European passerines and
they usually lay one egg per day, although laying interruptions are
frequent. We performed a lutein supplementation experiment, and measured
potential consequences for egg laying capacity and egg quality. We found
that lutein-supplemented females had less laying interruptions and thus
completed their clutch faster than control females. No effects of
treatment were found on the onset of egg laying or clutch size.
Experimentally enhanced carotenoid availability did not elevate yolk
carotenoid levels or egg mass, but negatively affected eggshell thickness.
Our results provide hence evidence on the limiting role of carotenoids
during egg laying. However, the benefits of laying faster following lutein
supplementation were counterbalanced by a lower accumulation of calcium in
the eggshell. Thus, even though single components may constrain egg
laying, it is the combined availability of a range of different resources
which ultimately determines egg quality and thus embryonic development.