Birds are pushed out of the nest

Home hygiene as an evolutionary advantage in the fight against "cuckoo children"

The cuckoo is not always successful when it tries to subjugate its eggs to another bird to incubate. Researchers from the Konrad Lorenz Institute for Comparative Behavioral Research at the University of Veterinary Medicine, Vienna, observed in the investigation of 60 feldsparrow nests that the birds recognize foreign eggs and throw them out of the nest. The scientists have now found out why they do this and whether innate cleaning behavior plays a role.

Building a nest, laying eggs, hatching and looking after the chicks are energy-intensive tasks for a bird. That is why birds are careful to really only invest in their own young. So-called nest parasites often do not make life easy for the bird parents. The strangers must be recognized and thrown out of the nest. If the birds cannot distinguish foreign eggs from their own, they run the risk that the foreign offspring will even kill their own.

Each bird species has its own strategy for dealing with "aliens" in the nest. For example, some birds count the eggs in the nest and thus recognize precisely when one is suddenly too many. Others orientate them more to the size or the pigmentation of the clutch. Some discover that a "cuckoo's egg" has been slipped under them, but they don't have the strength to push the strange egg out of the nest. Some birds don't even care about foreign eggs in the nest.

What causes the birds to throw eggs?

The field sparrow (Passer montanus) produces eggs of different sizes and colors within its species. Behavioral scientist Herbert Hoi and his colleagues from the Konrad Lorenz Institute for Comparative Behavioral Research wanted to know how well the feldspar can differentiate between its own and other eggs and whether it can also remove them from its own nest. To do this, the researchers placed real eggs, but also egg dummies in the form of egg-shaped discs in different colors and sizes, in the nests. The behavioral researchers chose these objects because they are easy to remove from the nest, even for small songbirds. There is no risk that sparrows cannot throw eggs out of their breeding grounds due to their insufficient physical strength.

The researchers wanted to know why the sparrows remove certain objects from the nests, how quickly they do so, when and why. The reasons why the field sparrows throw foreign eggs out of their nests has not yet been investigated. Therefore, at different times, they placed foreign eggs or egg dummies in the nests, which differed greatly from the eggs of the species.

Nest hygiene as a motivation

It turned out that field sparrows throw about a third of the foreign eggs out of the nest during the laying phase. They continued to hatch the remaining foreign eggs. More than 80 percent of the dummies were thrown out of the nests, regardless of whether they were during the egg-laying or the incubation phase. On the one hand, these results show that the sparrows can recognize foreign eggs and remove them from their nests. However, this behavior does not work 100 percent. Either the birds do not recognize all foreign eggs or they do not always manage to lift the heavy eggs out of the nests with their beak.

"We were surprised that the birds removed foreign objects from their nests more often during the laying phase than during the breeding phase. We believe that the field sparrows want to avoid hatching foreign eggs in this way. But there is another motivation: Keeping the breeding site clean. It was found that white and angular objects were removed less frequently during the breeding season. These shapes and colors probably remind the birds of eggshells. Usually, the animals remove these parts only after the chicks have hatched, thus keeping the nest clean for the offspring. " (red,, 05.01.2014)