An African bird that robs other species of their food seems to help as well as hinder, allowing one of its victims to catch more prey.

Pied babblers (Turdoides bicolor; pictured left) are often fooled by fork-tailed drongos (Dicrurus adsimilis; pictured right), which mix fake alarm calls with genuine warnings to distract the babblers, then make off with their food. Andrew Radford at the University of Bristol, UK, and his team studied the birds in the wild, and played back drongo recordings to babblers. They found that babblers captured more prey when reassured by the regular 'twank' noises of real or recorded drongos than when there were no drongos standing guard.

This 'sentinel' calling probably arose as a means of manipulating babblers, but the benefits it brings to both species may mean that the birds' relationship is transitioning from parasitic to mutualistic, the authors suggest.

Credit: A. RADFORD

Evolution doi:10.1111/j.1558-5646.2010.01180.x (2010)