A common cuckoo (Cuculus canorus) female lays her egg in the nest of another bird, and after it hatches, the cuckoo chick typically pushes out the host bird’s own eggs. But now, communities of reed warblers (Acrocephalus scirpaceus) have developed a "neighborhood watch" in their marshlands to keep would-be victims up to date with the latest parasitic cuckoo threats. According to a new Scientific Reports study, warbler parents combine personal experience with social information of local risk before pushing out the imposter egg. 

For cuckoos, it pays to be sneaky. Some resemble hawks, so it could be potentially dangerous for warblers to approach or mob them. And not only do female cuckoos lay eggs in other birds’ nests in as little as 10 seconds, but those cuckoo eggs also mimic the appearance of the hosts’ eggs. Previous work revealed that reed warblers become aware of the risks of parasitism from frequent personal encounters, and they also respond to social cues regarding local cuckoo activity. 

To test whether egg rejection depends on information from a variety of sources, University of Cambridge’s Rose Thorogood and Nicholas Davies conducted experiments with reed warblers during breeding season (May to July) in 2013 and 2014 on Wicken Fen in Cambridgeshire. This area has a low rate of natural parasitism: About 5 percent of the nearly 200 nests are parasitized a year. 

The team used playbacks of reed warbler alarm calls as well as wooden models of an adult cuckoo and a grass parrot (a harmless intruder serving as a control). To gather social information – that is, “there’s a cuckoo active in the local neighborhood” – the team placed the models in a neighboring territory that’s less than 40 meters (130 feet) away from a focal pair’s nest. For personal information – “the cuckoo has found my nest” – the models were adjacent to or even touching the rim of the focal pair’s nest. One of the four eggs in each warbler nest was painted with 40 small brown spots. 

They found that a combination of social information and personal information about local cuckoo presence is necessary for reed warblers to reject an egg from their nest. Either one by themselves weren’t sufficient, and control intruders produced no effect.

Personal experience alone may be unreliable because the presence of a cuckoo at the nest isn’t a sure predictor of parasitism: Cuckoos resemble hawks, and sometimes female cuckoos visit host nests just to inspect the clutch or remove a host egg. Similarly, social cues alone didn’t stimulate egg rejection because a female cuckoo wouldn’t parasitize every host nest in her territory. 

Additionally, pairs that rejected eggs weren’t more (or less) likely to mob a cuckoo – suggesting that mobbing and egg rejection are independent lines of defense. 

^{Image in the text: Wang LiQiang/Shutterstock}