More exciting or realistic comparisons will be tested in the lab with reared offspring or acclimated adults to exclude major environmental effects. Genetic differentiation on neutral markers versus selected traits can be ascertained in the project of Pieternella Luttikhuizen. Due to removal of mussel beds by human fisheries and a row of years with low recruitment and mass mortality of adult cockles, the availability of surface and shallow-buried bivalves has dramatically declined in the 90's. It is known that Oystercatchers with short bills specialize on shallow prey, whereas long-billed individuals select deeper living prey. Especially in cold winters, food availability of shallow prey may decline dramatically since adult cockles die quickly when it freezes.
Many Oystercatchers remain near the breeding grounds and hope for better weather.
This mortality may thus be selective, with longer-billed individuals having higher survival. There is large within-sex variation in the bill size of Oystercatchers, and this size seems to have a heritable component, as has been shown in the ringed population of Schiermonnikoog. Since this breeding population is very site-loyal and bill characteristics of all breeders are known, it will be possible to quantify the selective force on bill size of the reproducing population. The evolutionary response in the new generations can be calculated from these existing data. An even more exciting extension would be the adaptive response to natural selection on culturally transmitted traits.