Can social interactions rescue “lost” behavioural defences in an allopatric cuckoo host?

Abstract

Social information use by cuckoo-hosts provides an excellent example of how social interactions within species can influence selection on another. Neighbours observe each other's mobbing behaviour towards cuckoos, and then adjust costly defences back at their own nest in response. This may even allow naïve juveniles to 'learn' to recognise cuckoos, suggesting that 'social responsiveness' must play a key part in determining how hosts respond to changing rates of brood parasitism. However, if the 'cultural memory' of cuckoos is lost, can this plasticity in defences be rescued? This is fundamental for cuckoo-host coevolution, as the rates at which host defences are lost, gained, or retained determines evolutionary dynamics. Using populations of reed warblers at the northern- and southernmost extents of their range, we designed a field experiment to test whether populations allopatric to cuckoos (i) still recognise cuckoos as a threat, (ii) respond to social cues to upregulate defences, and (iii) vary in their strength of social responsiveness. We found defences and social responsiveness to be weaker in the south, where there is a much longer history of allopatry. In the north, only birds that retained recognition of cuckoos showed social responsiveness: social information about cuckoos did not increase hosts' propensity to attack (whereas it doubled in the range core), but it did increase the strength of mobbing in the 30% of birds that recognised the cuckoo at the northern range front. Most importantly, however, experimental cuckoo eggs remained in the nest, whereas in previous experiments elsewhere the same social information increased egg rejection ten-fold. This suggests that social responsiveness is more limited at the range edges, under relaxed selection, and this would incur little immediate cost to cuckoos' fitness if they (re)invade. Our study provides a rare example of geographic variation in social information use and highlights how current range expansions may influence the outcome of future species interactions.