"Extensive debate in the literature was first focused on the question: Why does a population's average clutch size differ from the most productive clutch size?" said researcher Ann Charmantier. "Gradually the debate switched to an evolutionary point of view with a second, related question: Why is clutch size not evolving despite significant heritability and directional selection?"

Many long-term studies of avian clutch size have looked for, but not seen, an increase in the number of eggs laid. However, this 25-year study of the selection, inheritance, and evolution in the mute swan population of Abbotsbury, England, yielded data on clutch size consistent with the direction predicted by evolutionary theory.

"In this study population, clutch size shows a clear response to selection, providing us with a clear illustration of microevolutionary process on a small timescale," explain the researchers.

The researchers hypothesize that a recent relaxation of food constraints and an increase in protection from predators may have helped enable the swans to evolve towards a new, larger clutch size.