BEHAVIORAL-RESPONSES OF POTENTIAL HOSTS TOWARDS ARTIFICIAL CUCKOO EGGS AND DUMMIES

Responses of 33 potential host species towards a non-mimetic, dummy, cuckoo egg placed in their nest were tested (N = 372). For 22 of these species, their behavioural responses towards a dummy cuckoo placed near their nest were also tested (N = 193). The species were grouped in A) most common hosts: species which at the moment are losing out in the coevolutionary arms race with the cuckoo and which today represent favorite hosts; B) frequently-used hosts: species which at the moment are assumed to be true cuckoo hosts, but which are not so commonly used as those in group A; C) rarely-used hosts: species which would appear to be suitable hosts, but which despite of this, are rarely used. These species are assumed to be ahead of the cuckoo in the coevolutionary arms race; D) unsuitable hosts: species with a breeding biology which either prevents, or counteracts, cuckoo parasitism. They are therefore assumed never to have been engaged in a coevolutionary arms race with the cuckoo. In the most common hosts the median acceptance rate of the non-mimetic egg was 86%, in the frequently-used hosts 33%, in the rarely-used hosts 10% and in the unsuitable hosts 100%. In the most common hosts the median rate of aggression shown towards the cuckoo dummy was 50%, but the most numerous species in this group, the meadow pipit, showed aggressive behaviour in 60% of the cases. The median aggression rate both in the frequently-used hosts and the rare hosts was 100% and in the unsuitable hosts 0%. The bluethroat was the only species which accepted the non-mimetic dummy egg at a higher rate later on during the incubation period than during earlier stages. A positive correlation was found between the power of egg discrimination and the rate of aggression shown towards the dummy cuckoo. Such aggression was stronger when both parents were present at the nest than when only one parent was present. The results of this study lend support to the hypothesis that the differences in the degree of responses by the host species towards parasitism by the cuckoo reflect different stages in a continuous coevolutionary arms race with cuckoos.