January 7, 2019,
Little more than 50 years after the German ornithologist Wolfgang Makatsch published his book titled No Egg Is Like Another (Kein Ei gleicht dem anderen), new research at the Max Planck Institute for Ornithology in collaboration with the Max Planck Institute of Biochemistry and the University of Hohenheim reveals exactly how right he was. The study describes for the first time the egg albumen and yolk proteomes—that is, all measurable proteins—of a common songbird, the blue tit. It shows that breeding females can fine tune their eggs' composition to the needs of their young.
The quality of an egg resides in its size and its composition. In all egg-laying species, larger eggs produce larger, more viable and more performant , while specific egg components (such as antioxidants or antimicrobial proteins) can support offspring growth, development or immunity. Egg quality varies broadly in wild animals and can even vary within a clutch and induce differences among siblings. To understand what drives this variation, researchers have turned to a population of blue tits (Cyanistes caeruleus) breeding in southern Germany. They measured the nutrient content of the eggs, as well as the individual concentrations of six carotenoids (adding three new compounds to the list of avian egg carotenoids) and almost 300 egg proteins, and explored how the complex composition of the eggs varies in the population.
Blue tits are small, socially monogamous songbirds that breed once per year. Females use the nutrients they acquire from their daily food to produce one egg per day until the clutch has seven to 15 eggs, and a total weight that often exceeds her own body mass. Such a reproductive endeavour is difficult to sustain, and larger clutches contain smaller eggs. Moreover, the capacity to acquire, synthesize and deposit egg nutrients varies among females, resulting in eggs with very different composition.