Eggy arms race

(Image: Claire Spottiswoode)

The cuckoo finch isn't a true cuckoo but it too lays lookalike eggs in the nests of other species, which hatch and are raised by an unsuspecting host parent.

The arrangement results in an arms race between the birds: cuckoo finches (Anomalospiza imberbis) evolve to become better at imitating host eggs, and hosts to become better at detecting the fakes.

A collection of eggs from wild cuckoo finches and one of its hosts, the tawny-flanked prinia (Prinia subflava), demonstrates just how quickly the arms race can drive evolution.

Over the last 40 years the prinia has responded to its invaders by evolving a wider spectrum of colours and patterning. The present-day sample of prinia eggs, shown in the outer ring, demonstrates this. The inside ring show the smaller range of colours and patterns produced by cuckoo finches.

Individual prinias lay very different-looking eggs, says Claire Spottiswoode of the University of Cambridge. This makes it less likely that a cuckoo egg will match. Prinias have also shifted their eggs toward the olive side of the palette over the last 40 years, a colour cuckoo finches currently find hard to imitate.

Spottiswoode expects that in future years the cuckoos may well adopt the olive palette in response - and in turn, prinia fashion may be on to some new hue.