In this study, Proceedings of the Royal Society B , researchers at the University of North Carolina and the Max Planck Institute replaced 36 nests with boxes. Each artificial nest contained an egg-laying; half of the nests were dry grass only. In the other half of the nests, dry herbs were mixed with other plants often found in starling nests : yarrow, large hogweed, chervil, black elder , angelica , willow.

The researchers also placed in the nest a false egg that recorded the temperature. They observed that egg temperatures and nesting numbers were higher in nests that had aromatic herbs , especially at the beginning of the incubation period. In addition, egg temperatures are less often below critical thresholds in nests containing aromatic herbs . These parents also started their active day earlier.

Herbs that influence parental behavior

The percentage of eggs that hatched successfully was equivalent in both types of nests, but the nesting nests with the aromatic herbs appeared to grow faster: one week after hatching, they were larger than the others. This can be explained by the fact that the heat brought by the parents favors the embryonic development.

But how did the herbs affect the parents? One hypothesis is that they have some kind of pharmaceutical effect. This is what Helga Gwinner, lead author of the study, suggests in a statement :  “Starlings select particular herbs for the decoration of their nests. Curiously, some of these herbs are also used in folk medicine. Their known sedative effects may influence incubation behavior by inducing increased nesting frequency and reducing egg exposure at low ambient temperatures. “ But it also adds that  ” More recently, the birds have also started to include human objects in their nests. “ Not sure that all these objects are very useful …