http://www.tau.ac.il/~lotem/pdf/Anim95.pdf

Constraints on egg discrimination and cuckoo-host co-evolution

An examination of about 12000 clutches of European passerines that contained eggs of the common cuckoo (Cuculus canorus), held in museum egg-collections, revealed statistically significant correlations between the cuckoo and the host eggs within a clutch in volume, ground colour, and size and percentage coverage of the spots. Although most cuckoo eggs were yellowish, the range in coloration and the percentage coverage of spots indicate that in these respects the cuckoo eggs are distributed along a continuum. However, a similar distribution was also found among the pooled host eggs. These results provide some support for the ‘host preference hypothesis’, which states that each cuckoo female specializes on one particular host species.

By using a subjective classification, we found that there are at least 15 different cuckoo egg-morphs in Europe, but only 44% of the clutches contained cuckoo eggs of the egg-morph corresponding to the host eggs. The ‘host preference hypothesis’ therefore cannot provide a satisfactory explanation for the variation found among cuckoo eggs in Europe. However, 77% of the cuckoo eggs had been laid in nests of hosts with nesting sites similar to those of the main host of the egg-morph. This indicates that cuckoos also parasitize several species whose nest sites are similar to those of their main host. These results therefore provide support for the ‘nest site hypothesis’ which states that each cuckoo female parasitizes a group of host species with similar eggs or nest sites. The ‘natal philopatry hypothesis’ which states that female cuckoos may search for nests completely at random in their natal habitat is only weakly supported and can probably be rejected.

Most of the hosts, main and secondary, nest among low vegetation or on the ground, whereas tree-nesting species are seldom parasitized. The most frequently used hosts in the egg collections we examined were species of the Acrocephalus and Sylvia genera of warblers, all species that breed in low vegetation.

Egg‐morphs and host preference in the common cuckoo (Cuculus canorus): an analysis of cuckoo and host eggs from European museum collections

Reed warblers sometimes make recognition errors when faced with a mimetic cuckoo egg in their nest and reject one or more of their own eggs rather than the foreign egg. Using the framework of signal detection theory, we analyse responses to model eggs to quantify the costs and benefits of acceptance versus rejection in parasitized and unparasitized nests. We show that below a threshold of 19-41% parasitism, the warblers should accept mimetic cuckoo eggs because the costs of rejection outweigh the benefits, whereas above this threshold they should reject. The warblers behaved as predicted; when they saw a cuckoo at their nest they usually showed rejection, but without the sight of the cuckoo they behaved appropriately for the average parasitism rate in Britain (6%) and tended to accept.

Recognition Errors and Probability of Parasitism Determine Whether Reed Warblers Should Accept or Reject Mimetic Cuckoo Eggs

I suggest that the cuckoo's parasitic adaptations are of two kinds: ‘trickery’, which is how adult cuckoos and cuckoo eggs and chicks evade host defences, and involves adaptations that have co-evolved with host counter-adaptations, and ‘tuning’, which is how, once accepted, cuckoo egg and chick development are then attuned to host incubation and provisioning strategies, and which might not always provoke co-evolution. Cuckoo trickery involves adaptations to counter successive lines of host defence and includes: tricks for gaining access to host nests, egg trickery and chick trickery. In some cases, particular stages of host defences, and hence their corresponding cuckoo tricks, are absent. I discuss three hypotheses for this curious mixture of exquisite adaptation and apparent lack of adaptation: different defences best for different hosts, strategy blocking and time for evolution of defence portfolios. Cuckoo tuning includes adaptations involving: host choice and monitoring of host nests, efficient incubation of the cuckoo egg, efficient provisioning and protection of the cuckoo chick, and adaptations to avoid misimprinting on the wrong species. The twin hurdles of effective trickery in the face of evolving host defences and difficulties of tuning into another species' life history may together explain why obligate brood parasitism is relatively rare.

https://zslpublications.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-7998.2011.00810.x

Cuckoo adaptations: trickery and tuning

Coevolutionary theory predicts that the most common long-term outcome of the relationships between brood parasites and their hosts should be coevolutionary cycles based on a dynamic change selecting the currently least-defended host species, given that when well-defended hosts are abandoned, hosts will be selected to decrease their defences as these are usually assumed to be costly. This is assumed to be the case also in brood parasite-host systems. Here I examine the frequency of the three potential long-term outcomes of brood parasite-host coevolution (coevolutionary cycles, lack of rejection, and successful resistance) in 182 host species. The results of simple exploratory comparisons show that coevolutionary cycles are very scarce while the lack of rejection and successful resistance, which are considered evolutionary enigmas, are much more frequent. I discuss these results considering (i) the importance of different host defences at all stages of the breeding cycle, (ii) the role of phenotypic plasticity in long-term coevolution, and (iii) the evolutionary history of host selection. I suggest that in purely antagonistic coevolutionary interactions, such as those involving brood parasites and their hosts, that although cycles will exist during an intermediate phase of the interactions, the arms race will end with the extinction of the host or with the host acquiring successful resistance. As evolutionary time passes, this resistance will force brood parasites to use previously less suitable host species. Furthermore, I present a model that represents the long-term trajectories and outcomes of coevolutionary interactions between brood parasites and their hosts with respect to the evolution of egg-rejection defence. This model suggests that as an increasing number of species acquire successful resistance, other unparasitized host species become more profitable and their parasitism rate and the costs imposed by brood parasitism at the population level will increase, selecting for the evolution of host defences. This means that although acceptance is adaptive when the parasitism rate and the costs of parasitism are very low, this cannot be considered to represent an evolutionary equilibrium, as conventional theory has done to date, because it is not stable.

Long-term coevolution between avian brood parasites and their hosts.

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.

Lars Korsnes

Papers

Behavioural Responses of Potential Hosts Towards Artificial Cuckoo Eggs and Dummies

Rejection of cuckoo (Cuculus canorus) eggs by meadow pipits (Anthus pratensis)