Reed warblers developed neighborhood watch to defeat cuckoos

Reed warblers, small passerine migratory birds that breed across Europe and western Asia, combine information from multiple sources in order to avoid being tricked by cuckoos, a new study co-authored by Branco Weiss fellow Rose Thorogood shows. The findings were published recently in Scientific Reports and were covered in the media including The Telegraph.

If a reed warbler detects a cuckoo egg in its nest, it can eject the odd egg. But the decision to remove an egg is tricky – cuckoo eggs look very similar to those of reed warblers, and cuckoos visit a nest for just seconds when they lay. Yet reed warblers, like many animals, are able to fine-tune their behavior to match threat levels closely. The new study now reveals how: reed warblers assess levels of risk by gathering information from a variety of sources.

The researchers used a series of controlled experiments in the wild, involving model cuckoos and broadcasts of reed warbler alarm calls, to find out more about how reed warblers learn from each other. They found that warbler pairs ejected an odd egg only when there was strong evidence that it might not be one of their own. For action to be taken, the clues had to add up. The warblers needed to be alerted by their neighbors’ behavior that there was a cuckoo at large in the neighborhood in addition to seeing a cuckoo near their own nest.

“Neither personal encounters nor social encounters alone were sufficient to stimulate egg rejection. Instead, information was combined from both sources. This is fascinating because we have assumed previously that animals favour one type of information over the other”, Thorogood said.

In the past 30 years cuckoo numbers have fallen across Europe, and in the region covered by the study they have declined by as much as 60 percent. As a consequence, the number of warbler nests used by cuckoos has also declined dramatically from between 10 to 20 percent to around 2 percent today. Today, reed warblers are much less likely to eject an egg from their nest than they were in the 1980s. This new study shows that reed warblers are not forgetting about the cuckoos, even when they are rare. Instead, reed warblers are using information to match their behavior to the changing level of risk.

Read the paper in Scientific Reports

Read the University of Cambridge’s press release

Read the article on The Telegraph

Read the article on IFLScience