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Brood parasitism by the common cuckoo Cuculus canorus has excited wonder, interest and speculation like few other bird behaviors. Its a species which had been studied and observed well before Darwin's time, generating a wealth of information that has provided a deeper understanding of interactions among species. However, recent studies are bringing up more details of the coevolutionary relationship between the cuckoo and its hosts. The aim of this paper is threefold: to provide a concise review of the evolution of brood parasitism in the common cuckoo, to outline the different strategies that host species have evolved to deter parasitism, and to identify some of the counteradaptations the cuckoo has developed against those strategies.
The extraordinary plasticity in breeding behavior of the family Cuculidae has no parallel among the world's bird families. Of the 136 species of cuckoos, 83 provide parental care while 53 species parasitize nests, where eggs are incubated and parasite's young are raised by a host species . Of those 53 parasitic species, the common cuckoo Cuculus canorus (hereafter, cuckoo) is the most widespread in Europe, inhabiting a diverse array of habitats and having at least 15 different host-specific races . Throughout history the cuckoo has attracted a wealth of attention and generated much fascination. In the fourth century BC, Aristotle accurately described the parasitic habits of the cuckoo, noting that "the nestling cuckoo ejects the host's eggs or young, casting out of the nest those with whom it has so far lived" . Despite these very early accounts of the cuckoo's breeding behavior, it was not until the 1700's that cuckoo's biology regained attention by scientists. However, the cuckoo literature generated prior to the mid-twentieth century was a mix of speculative observations and misconceptions. For instance, well into the 1750's it was thought that the female cuckoo upon laying the egg will carry it with her beak to the host's nest . Such an idea was mistakenly reinforced by observing cuckoo females carrying an egg, which later was proven to be the host's eggs, but assuming it was the cuckoo's egg. . This idea was later rejected by British physician Edward Jenner whose accurate observations led to the current description of the cuckoo's parasitism . Generally, cuckoos lay a single egg in a host nest. Upon hatching, the young cuckoo pushes out the host nestlings, and any unhatched eggs. The cuckoo nestling then demands undivided care from its hosts with an extraordinary begging display, which consists of bright orange gape and rapid calls to simulate several nestlings .
Evolution of cuckoo parasitism
It was Charles Darwin who first intended attended to explain the evolution of brood parasitism in European cuckoos. In The Origin of Species Darwin described an example of brood parasitism by an American cuckoo species. The cuckoo was reared by a Blue Jay Cyanocitta cristata until it fledged the nest. This observation led Darwin to suggest that brood parasitism has evolved through a repetitive process in nature in which the young cuckoo inherits its mother's parasitic habits due to advantages gain in survival . Darwin proposed that cuckoo parasitism evolved from parental cuckoos and phylogeny studies have supported his view. However, there is no consensus on how brood parasitism evolved. Taxonomists suggest that brood parasitism evolved as a separate event in two subfamilies of brood parasites, independently, the Old World Cuculinae and the New World Neomorphinae . On the other hand, a phylogeny study based on bone characteristics suggests that brood parasitism evolved as a single event in the family, placing Cuculinae and Neomorphinae together, while the genus Coccyzus found in the New World, re-evolved parental care from a parasitic ancestor . Yet another study on molecular phylogeny proposes three independent origins of brood parasitism, in genus Clamator, and other cuckoos of the Old World and cuckoos in the American continent .
Avian brood parasitism between distinct species is currently thought to have evolved from an ancestral line with parental care breeding or as an indirect consequence of brood parasitism between conspecifics . However, while the argument still exists over which of the the two pathways leads to the parasitism of other species nests, there is greater support for the evolution of parasitism from a normal breeding behavior [1,3]. It is considered that the ancestral form of modern day cuckoos probably was a parental care, non-migratory inhabitant of tropical forest, and that changes in habitat, migration patterns, range size and foraging provoked the evolution of brood parasitism as a way to reduce order the cost of reproduction [1.3]. When comparing energy expenditure on reproduction, it was estimated that a parasitic cuckoo spends half the amount of energy of a parental cuckoo in achieving comparable reproductive success. In fact, within the Cuculidae family there are important dissimilarities between species with parental care and brood parasites. Cuckoos have a lower body mass, lay smaller eggs, and feed on prey of lesser size. . In the comparative analysis done by Krüger and Davies (2002), it was concluded that the evolution of cuckoo parasitism was the result of changes in the ecology of a parental ancestor and that once a parasitic behavior was adapted further specializations occurred.
a. From raising your own offspring to be a brood parasite
Before brood parasitism evolved, there were key changes in the ecology and behavior of forest cuckoos. Initially, forest cuckoos expanded their range to include more open and seasonal habitats. This habitat expansion provoked a more migratory behavior, an increase in breeding-range size and the availability of new food sources . These changes in ecology required a reduction in energy invested in reproduction. When comparing energy expenditure on reproduction, it was estimated that a parasitic cuckoo spends half the amount of energy of a parental cuckoo in achieving comparable reproductive success. Therefore, selection for brood parasitism became the emerging consequence of the ecological changes mentioned above.
b. Brood parasitism caused the evolution of specialized parasitic adaptations
Upon the adaptation of obligated brood parasitism, cuckoo eggs became smaller. Kruger and Davies (2002) suggest two hypotheses for the reduction in egg size. First, parasitic cuckoos were able to increase clutch size which may have been possible upon the reduction of overall egg size. Since most hosts are smaller than their parasitic cuckoos, an adaptation to a decrease in egg size would have been selected in order to avoid host rejection while improving incubation efficiency. A preference for smaller hosts increases the number of egg-laying opportunities since there is a greater abundance of smaller species than larger ones  .
An adaptation to a parasitic behavior might have also freed the cuckoo of the diet restraint when selecting breeding habitat. By relying on the host parental care, cuckoos are able to exploit a diversity of breeding habitats for breeding even those habitat where adult cuckoos cannot forage. Kruger and Davies (2002) note that some common cuckoos are required to travel large distances when breeding in marshlands while foraging in woodland areas. This independence from breeding habitat restraints have enabled the common cuckoo to increase its range through Europe.
Factors that affect brood parasitism by the Common Cuckoo
The size of the host and its abundance is an important predictor of host parasitism . There is a greater diversity and abundance of small bird species; which provides the cuckoo a larger pool of possible hosts. The density of the cuckoo at a given area also plays a crucial role. It was found that rejection rate increase as the rate of parasitism goes up in a given area . A greater density of female cuckoos at the breeding area of the selected hosts increases the chances that the hosts see a cuckoo female and proceed with egg rejection or nest desertion . This might be one of the multiple explanations for the fact that cuckoos are more widespread and their breeding-range is significantly larger than parental species of the same family. Additionally the choice of nesting site affects brood parasitism. Nests located nearby trees have a significant increase on the risk of being parasitized . Female cuckoos perch on trees to gather information on the host's nest building activity and therefore the availability of visual points are crucial for the cuckoo to determine host's nest location . Therefore, it is expected of the cuckoo's hosts to show a high degree of plasticity in choice of nest site in order to decrease the distance to visual vantage point and to increase nest concealment . Interestingly, female cuckoos seem to gather cues from the host behavior not only to determine nest location but also to reveal the host's parental capacities. Males that sing more actively during breeding season tend to attract females that built larger nests. It was found that in great reed warblers Acrocephalus arundinaceous nestlings raise in larger nests receive more feeding than those in a small nest . It will be expected to see a larger rate of parasitism in hosts that are more vocal and active during nest construction. However, it was found that hosts with a high degree of signal expression nearby the nest are also more eager to defend the nest against intruders and to reject cuckoo eggs. Therefore, cuckoos may favor a host with a smaller nest and whose activity is less visually conspicuous . Polacikova et al. (2009) also found that great reed warblers affected by cuckoo parasitism have higher body condition than individuals freed of parasitism. Additionally, females hosts had higher body mass and presented less uniformity in egg coloration 
Host defensive strategies against cuckoos parasitism
Brood parasitism by the cuckoo brings multiple costs to the host . The primary cost is the loss of an egg once the female cuckoo lays hers. Additionally, rejection requires the expense of energy and time (for recognition) and it includes the risk of discriminating the host's own eggs. By accepting the cuckoo's egg the reproductive success of the host is then jeopardize since upon hatching the young cuckoo pushes out the host's nestlings . Furthermore, the host parents must raise a much larger chick which demands more food than the host's own nestlings. This costly is highly increased in the extraordinary case when the host and cuckoo nestlings grow up together like in the parasitism of the Redstart Phoenicurus phoenicurus . Due to the multiple costs associated with brood parasitism, many host species have evolved adaptations to cope with parasitism; such as egg rejection and nest desertion . However, these adaptations only partially offset the costs of parasitism since they take place once the nest has been already parasitized. Therefore, some host species have also evolved adaptations to eliminate laying opportunities for the cuckoo. Nesting in safe places, constructing well hidden nests, or an active protection of the nest from intrusions are example of adaptations that host species have evolved to avoid being parasitized [10,11].
The reed warbler Acrocephalus scirpaceus is one of the species most readily selected by cuckoos as host for their eggs and therefore has evolved adaptations that demonstrate a coevolutionary arms race with the common cuckoo [12,13]. Mobbing, the attacking or harassing of cuckoos by reed warblers has been determined to be an effective first line of defense against parasitism . It has been well documented that reed warbler use audible and visual displays that in occasion lead to direct physical attacks to dissuade parasitism. Such defensive behavior reduces the probability of parasitism and it is a direct cost to cuckoos which might lose feathers or suffer injuries during physical attacks by reed warblers . Additionally, mobbing attracts predators, and other brood parasites thus posing an indirect cost since the cuckoo or its eggs might be at risk of predation . The costs inflicted on the cuckoo can also be associated to the costs suffer by the reed warbler that mob brood parasites. First of all, there is a physical similarity between common cuckoos and sparrowhawks Accipiter nisus , a potential predator of reed warblers. Therefore, reed warblers may need to spend time to identify the type of threat, parasitism or predation, before engaging in a mobbing display . In addition, the same indirect costs that mobbing brings to the cuckoo are also to the reed warbler which unknowingly might be attracting more parasites and predators to the nesting area . Therefore, the costs inflicted on the host select for a defensive behavior that discriminates between the type of threat and the degree of parasitism . Although some passerine birds will mob a possible predators, nesting adult reed warbler have not been observed doing it. Welbergen and Davies (2008) note that adult reed warblers seek cover and remain silent in the presence of a sparrowhawk near the nest . Since sparrowhawks are a direct threat to the adults while cuckoos are to the nest, it makes sense that reed warblers mob the latter and not the former . Reed warblers also show plasticity in their mobbing behavior in terms of risk of nest parasitism. In areas where there is a higher probability that the cuckoos are able to find hosts nest, mobbing is the best strategy, while in areas with a low probability the best defensive method may be to avoid active and visible displays and remain hidden but alert . By presenting reed warblers with taxidermic mounts of cuckoos, Welbergen and Davies (2009) concluded that mobbing is an adapted, phenotypically trait with high plasticity in the defense against parasitism .
Alarms Calls and Nest Guarding
It was previously noted that cuckoos and sparrowhawks share a resemblance that might pose a challenge for reed warblers in their nest defensive strategies. However, reed warblers have shown to vary their auditory displays accordingly to the kinds of danger and the subsequent response by conspecifics . Welbergen and Davies (2008) showed that reed warblers are able to identify cuckoos from sparrowhawks with alarm calls that attract not only mates but nearby neighbors. The alarms signals by reed warblers in the presence of a cuckoo are characterized by rasps and snaps. Rasps have a sharp commencement and a wide frequency range, which are important features that enable the location of the emitter . Alarms calls therefore are an important communal defensive mechanism against parasitism. Nest attendance and egg rejection by reed warblers might increase due to the information provided by the alarm calls emitted by neighbor conspecifics . Additionally, alerting on the presence of cuckoos might trigger the mobbing of the parasite by multiple reed warblers which could eject the cuckoo. Nest guarding has also been identified as a defensive strategy against parasitism. However, it is more passive and less conspicuous than mobbing and alarm displaying . An increase in nest guarding has been observed after laying has begun and is usually done by the male. This behavior might able the reed warblers to gain information about cuckoos presence and chance of parasitism. Spotting a cuckoo before laying has begun might trigger nest desertion whereas the sighting of a cuckoo once laying commences might increase the likelihood of rejection of the cuckoo's egg .
Hosts species have evolved two mechanisms in order to eject cuckoo eggs. In the case of hosts with large beaks, the eggs are grasped and evicted out of the nest while species unable to grasp the egg, puncture and then eject the parasite's egg . The reproductive success is minimal if a host accepts a cuckoo egg due to the innate behavior of the cuckoo nestling of evicting host's eggs and nestlings. Therefore, species parasitized by cuckoos should strongly select for an ejection adaptation to parasitism if the host is physically able of ejecting cuckoo eggs . For those species whose beak is too small to grasp the beak and that puncturing the egg would be too costly or impossible, nest desertion is the strategy selected once parasitism has been identified [14,15]. The cost to the hosts when trying to puncture cuckoo eggs has been demonstrated in marsh warblers Acrocephalus palustris which after unsuccessfully attempting to puncture the eggs have damaged their own eggs .
Cuckoo's Responses to Overcome Host's Adaptations
Cuckoo parasitism has led to the selection of defense mechanisms by the hosts. At the same time, in this coevolutionary arms race, more sophisticated trickeries are selected by the cucook [1,16]. Egg mimicry and vocal mimicry by the nestlings are of the most significant traits evolved in the cuckoo to cope with host's defensive responses.
The recognition and ejection of cuckoo eggs have selected for an increase in mimicry of the host egg by the cuckoo whose eggs display a high variability in color and/or spotting due to the diverse number of hosts it parasitizes . In a reed warbler study of parasitism by the common cuckoo, Avilés et al. (2006) found the surprising speed in which egg similarity between the two species evolved in recently parasympatric poputions of reed warblers and cuckoos. Using museum egg collections, it was found that in 23 years, the degree of egg matching increase considerably . Avilés et al. note this rapid improvement egg matching a micro-evolutionary response to host removal of eggs differing in color or size and that egg mimicry has coevolved with this rejection behaviour . The importance of egg mimicry was exemplified by Antonov et al. (2008) on a study of cuckoo egg rejection by marsh warblers. On the study, unmanipulated cuckoo eggs were accepted more readily than painted cuckoo and great reed warbler eggs placed in marsh warbler nests . The aim of the Antnovo et al. was to determine the importance of egg shell strength in deterring egg rejection. However, since painted cuckoo eggs suffered a higher rejection rate, it was indicated that egg mimicry is pivotal in determining the probability of rejection .
Vocal or Visual Mimicry by Parasite Nestlings
Brood parasites can be differentiated into two categories: 'Nonevictors' which are parasites that grow up alongside the host's young and 'evictors' in which the young parasite either kills the host's nestlings or expels them out of the nest along with any unhatched eggs . Nonevictor species include the brown-headed cowbird, Molothrus ater, finches of the Vidua genus and great spotted cuckoo, Clamator glandarius. While the common cuckoo is a evictor brood parasite. Nonevictors showed certain mimicry of the host chicks as a counteradaptation against the host adult learning of the characteristics of the chicks during first brood. These imprinting of its own chicks characteristics will later permit the rejection of chicks that are different . For example, Vidua finch young show great resemblance of the mouth spot patterns of their host's chicks . On the other hand, this adaptation has not been selected in hosts of evictor parasites. Since the cuckoo nestling is raised alone, imprinting the characteristics of the parasite chick will be detrimental to future broods . Butchart et al. (2003) found that the begging calls produced by four different common cuckoo races did not differ which lead to the conclusion that in evictor species there is not selection for evolution of visual or vocal mimicry by the parasite chick . However, it should be advantageous for the cuckoo nestling to respond to the host alarm calls given in the presence of a predator . Davies et al. (2006) concluded that cuckoos that specialize on reed warblers have not only well-matched eggs but also chicks that are well-tuned to the host's alarm calls .
Coevolution relationship among species are significant in order to understand how species select for adaptations accordingly to the other species. In the case of brood parasitism by the common cuckoo, that relationship is truly an arms race. For instance, egg rejection lead to egg mimicry which in turn might lead to clutch variability in the host. However, before egg rejection is selected, hosts have evolved a first line of defense that includes direct confrontation against the parasite. The costs of parasitism are evident for the host species but also the parasite carries its own costs. As a relation observed well before Darwin and his theory of evolution, the cuckoo-host interaction was provided with a solid framework to be better study and understood once Darwin's work became public. However, necessity for further research remains. There is limited work on the biology and ecology of cuckoo nestlings. Also there is still no detailed explanations on the failure of hosts to recognized cuckoos' young as a different species . Furthermore, as anthropogenic actions change natural ecosystems at a rapid scale, further research is needed in the relationship between cuckoos and hosts and the factors affected by habitat alterations. .