Testing for correlations between behaviours in a cuckoo host: why do host defences not covary?

Fitness costs associated with brood parasitism have led host species to evolve several lines of defence. The first two lines of defence, aggression against adult parasites and egg rejection, are present at varying levels in almost all hosts. However, it remains unclear how these two fundamental defences covary at host individual level, with previous studies suggesting both positive and negative correlations. A theoretically critical yet empirically untested scenario is that variation in host antiparasite behaviour may relate to individual variation in host behavioural types or personalities. Here we examined whether host aggression against adult brood parasites and egg rejection behaviour were correlated with host behaviours displayed outside the context of brood parasitism. We selected the great reed warbler, Acrocephalus arundinaceus, a favourite cuckoo, Cuculus canorus, host as a suitable model. Only females reject foreign eggs and show high individual repeatability of both aggression towards cuckoos and nest guarding. We found that female behaviours in different situations (nest guarding, nest defence, handling in the net) were strongly correlated with each other. This is the first empirical evidence on correlation between individually consistent antiparasite adaptation (female nest defence) and behaviours that are not directly related to brood parasitism. In contrast, egg rejection/acceptance responses and latency to these responses did not correlate with any of the female defence/guarding behaviours and behaviour during handling. Proximately, this may be because nest defence and egg recognition represent cognitively and behaviourally completely different tasks. These patterns were not affected by female mating status in this polygynous cuckoo host. We hypothesize that differences in host behavioural types, rather than host egg discrimination ability, may predict host nest defence behaviour against adult brood parasites in general.