Mother Nature is capable of remarkable phenomena across every biosphere, including vivid and emotive displays, colourations, diversity and interactions. However, a newly hatched fledgling pushing intact eggs out of its own nest is a sight to behold.
Going Cuckoo: Parasites of the Avian World
Biology & Zoology | Ella Speers
Cuckoo birds, Cuclidae, are parasites scorning the systematic operation of nature. Through years of evolution manifesting into cheating tactics, they have freed themselves from the cost of parental care by inflicting this on a host species instead. A cuckoo will lay an egg in a host species’ nest when vacant to trick it into raising its young as part of its own brood, thus escaping the energetic expense of parental care.
However, nature is not that straightforward. The cuckoo’s cheating tactics are mirrored by the host species’ own evolutionary leaps in an attempt to rid itself of the parasitic cuckoo and the expense of raising its young. It is a cyclical process, a typical pattern in nature. Over periods of evolutionary time, the cuckoo will evolve an adaptation to trick the host, the host will develop a defence to block this, the cuckoo will create another adaptation, only to again be overcome by the host, and on the cycle goes. These rapid and intensive cycles of co-evolutions by parasite and host in response to selection pressure from the other are the epitome of an evolutionary arms race [1].
The cheating mechanisms that cuckoos use to inflict a host bird into raising its own young have earned them the title of obligate brood parasites. This parasitism has evolved independently three separate times within the cuckoo family [2].
The parasitic adaptations that a cuckoo uses to exploit its host are a form of either trickery or tuning.
Cuckoo trickery includes an exhaustive list of remarkable adaptations that have arisen to overcome host defences, such as host-egg mimicry and developing stronger egg shells that are resistant to damage by the host. Trickery ultimately aims to evade the hosts’ defences and to trick the host into raising the cuckoo egg as one of their own [2].
Cuckoo Trickery Versus Tuning
In comparison, tuning strategies ensure that the cuckoo egg and subsequent chick development are suited to the host species’ life history strategies to give it the best chance of survival [2]. A range of specific adaptations is required to ensure cuckoo development is conducive to the host's niche, such as its incubation and provisioning strategies, which have evolved to suit the host’s life history, not the cuckoo’s [2].
The first example of trickery exhibited by a cuckoo is exhibited by its access to hosts’ nests. A female may invest a significant amount of time observing a host bird from a concealed perch [2]. This will give the cuckoo insight into how the host behaves, including its usual feeding patterns and when the nest is not guarded.
Raising parasitic young is extremely expensive as it reduces the clutch size and success of the fledglings of the host’s brood [3]. To reduce the chances of their nests being seen and therefore exploited, host species employ a range of strategies which may include nesting further from sites where cuckoos have been seen to cryptically perch [2], concealing their nests [4], secretive behaviour, or unpredictable laying; methods which make timing the parasitism difficult for the cuckoo. Perhaps the most elaborate method of avoiding cuckoos, hosts may alter their nest architecture by narrowing the entrance tubes into their nests, so the bigger cuckoos will struggle to enter [5].
Cuckoo Trickery in Accessing Host Nests
A host may attack an approaching cuckoo through mobbing [2]. A previous study by Welbergen and Davies [6], shows that hosts who mob approaching cuckoos more aggressively were less likely to become parasitized, giving hosts an incentive to attack intruders they recognize as cuckoos.
Cuckoos aim to remain as cryptic as possible, as other hosts in the area will increase their attendance at their nests and rates of egg rejection when cuckoos have been identified in their areas [7], hence cuckoos may be inclined to avoid species they remember as being strong mobbers to avoid injury risk and attracting predators or other brood parasites [8].
To overcome hosts’ nest defences, cuckoos employ secretive behaviour and rapid laying [2]. They also benefit from plumage that resembles predatory hawk species [9], as predator resemblance allows cuckoos fitness benefits through attack avoidance by hosts [10]. Furthermore, to counter the ability of visual recognition that host species may possess, some cuckoo species have polymorphic female plumage – the existence of two or more different colour morphs over different time periods. By employing this strategy and essentially changing their appearance, cuckoos become unidentifiable to hosts and can therefore exploit hosts’ resources to raise their young.
Host Nest Defence
While cuckoo trickery for access to host nests shows immense strategic evolution, cuckoo egg trickery is an even more complex and sophisticated mechanism. Host adaptations (egg rejection) select for parasite resistance (egg mimicry) in an intricate co-evolutionary arms race [11]. The similarity of eggs between the common cuckoo and those of hosts was first noted in the mid-18th century [12]. Research since this period has revealed that some species can recognize their own eggs. In a nest of eggs, a host’s egg may serve as a reference for the egg type that is the correct one (its own), and hence provide a template for deducing which are foreign [13].
Species of birds with no history of cuckoo parasitism showed no rejection of foreign eggs, as demonstrated in Davies and Brooke’s 1988 study [7]. In comparison, previous hosts of cuckoo parasitism did reject eggs that were unlike their own. Davies and Brooke here show that egg rejection by hosts evolves in response to cuckoo parasitism.
Conversely, but also contributing to the arms race of evolution, cuckoo egg mimicry evolves because of host egg rejection [2]. For example, reed warblers (Acrocephalus scirpaceus) reject eggs that differ from their own, so their cuckoo parasite produces a mimetic egg. Strengthening this concept but through the opposite mechanism, dunnocks (Prunella modularis) do not discriminate on different eggs; hence, their cuckoo host lays a non-mimetic egg in these nests.
Egg Trickery
Some cuckoo species will eject the host’s eggs or kill the host’s young to enhance their own survival success [14]. While having an egg size that closely matches the size of the host’s eggs, these cuckoos, known as ejectors, parasitize hosts that are smaller than themselves to allow a newly hatched cuckoo to push the unhatched host eggs out of the nest [14]. Ejector cuckoos have, therefore, evolved a smaller egg for their body size to facilitate this phenomenon. Darwin [15] suggested the small egg size was advantageous in both deceiving foster parents into thinking it was their own egg, and hatching within a shorter period to promote the pushing of other unhatched eggs out of the nest.
Cuckoo Chicks Favour Their Own Survival
It has been a great mystery to zoologists as to why hosts of cuckoo parasitism exhibit discrimination against eggs unlike their own, yet some will accept a cuckoo chick upon hatching [2]. In species where the cuckoo is a non-ejector and is raised alongside the host’s brood, this is especially hard to understand, since a cuckoo chick tends to be larger and have a different gape flange colour when compared to the host’s fledglings. These two cues of size and colour are precisely what is employed in egg discrimination [7], so it is difficult to understand why these cues cannot be used to differentiate between chicks too. A theory for the acceptance of chicks, proposed by Davies and Brooke [7], is that eggs look the same during the incubation period. In contrast, chicks change dramatically in appearance from day to day. Identifying a foreign chick may pose a challenge in a clutch of constantly changing chicks.
In hosts that do reject foreign young, co-evolutionary theory accurately predicts that cuckoo parasites have evolved a visual mimicry of the host’s chicks’ nestling down, skin colour and gape flanges [2] that are employed to prevent cuckoo chicks from being rejected.
Chick Trickery
Tuning of a cuckoo egg and the subsequent chick is a recent proposal that requires more research. Current findings suggest that tuning first begins with host choice, explicitly finding a host that has a suitable size, diet, and nest type for a cuckoo chick. For ejector cuckoo chicks, the nest cannot be too deep to prevent the successful ejection of the host eggs [16]. Parasitic cuckoos are likely to need cognitive ability to allow them to remember the spatial and temporal availability of suitable host nests [17], hence females of the Molothrus species exhibit a larger hippocampus region than males [18].
A suite of adaptations ensures cuckoos hatch before the host’s eggs so that ejectors can expel them from the nest. For non-ejectors, hatching first allows a head-start in development, and hence a greater chance of out-competing host chicks [2].
In cuckoo chicks, tuning requires a further suite of adaptations that differ from the egg’s. These will vary, depending on whether a chick is an ejector.
In ejectors, the cuckoo is raised alone and receives all the food its host parent brings to the nest. It therefore simply needs to ensure the host brings enough food, although the usual host-specific fledgling strategies of begging for more food cannot be employed, as there are no host chicks to learn these off [2]. To compensate for this, a cuckoo chick will employ extravagant begging signals to increase host provisioning, such as rapid begging [19] and wing patches to stimulate extra gapes in the nest [20].
Non-ejector cuckoo chicks can use the other chicks to solicit food, so they tolerate the host’s chicks. However, they then have to compete for this on delivery [2]. Through tuning strategies, a cuckoo in a nest of host fledglings will take the most food by stretching higher, begging most intensively, and manipulating the hosts into favouring it over their own young [21].
Cuckoo brood parasitism is an extraordinary phenomenon that has fascinated zoologists for centuries. Trickery, the refined art of cheating, involves adaptations evolved to counter host defences, leading to remarkable coevolutionary arms races in both parasite and host to overcome the other.
Tuning may allow hosts to escape parasitism through evolutionary changes in their life-history strategies as cuckoos learn them. However, these are likely to occur on significant temporal scales, and immediate behavioural defences may suffice.
Obligate brood parasitism yields an interaction between two species that is a wonder of the animal kingdom.
Cuckoo Tuning to Host Life Histories
[1] R. Dawkins, J. R. Krebs, J. Maynard Smith, R. Holliday, “Arms races between and within species,” vol. 205, no. 1161, pp. 489–511, Sep. 1979, doi: 10.1098/rspb.1979.0081.
[2] N. B. Davies, “Cuckoo adaptations: trickery and tuning,” vol. 284, no. 1, pp. 1–14, 2011, doi: 10.1111/j.1469-7998.2011.00810.x.
[3] M. Hauber and K. Montenegro, “What are the costs of raising a brood parasite? Comparing host parental care at parasitized and non-parasitized broods,” vol. 10, pp. 1–9, Jan. 2002, Accessed: Aug. 21, 2022. [Online].
[4] C. Moskat and M. Honza, “Effect of nest and nest site characteristics on the risk of cuckoo Cuculus canorus parasitism in the great reed warbler Acrocephalus arundinaceus,” Ecography, vol. 23, no. 3, pp. 335-341, Jun, 2008, doi: https://doi.org/10.1111/j.1600-0587.2000. tb00289.x
[5] S. Freeman, “Egg variability and conspecific nest parasitism in the Ploceus weaverbirds,” vol. 59, no. 2, pp. 49–53, 1988, doi: 10.1080/00306525.1988.9633694.
[6] J. A. Welbergen and N. B. Davies, “Strategic variation in mobbing as a front line of defense against brood parasitism,” vol. 19, no. 3, pp. 235–240, Feb. 2009, doi: 10.1016/j. cub.2008.12.041.
[7] N. B. Davies and M. de L. Brooke, “Cuckoos versus reed warblers: adaptations and counteradaptations,” vol. 36, no. 1, pp. 262–284, 1988, doi: https://doi.org/10.1016/S0003- 3472(88)80269-0.
[8] J. N. M. Smith, P. Arcese, I. G. McLean, “Age, experience, and enemy recognition by wild song sparrows,” Behav Ecol Sociobiol, vol. 14, no. 2, pp. 101-106, Feb, 1984.
[9] R. B. Payne, “Interspecific Communication Signals in Parasitic Birds,” vol. 101, no. 921, pp. 363–375, Sep. 1967, doi: 10.1086/282504.
[10] O. Krüger, N. B. Davies, M. D. Sorenson, “The evolution of sexual dimorphism in parasitic cuckoos: sexual selection or coevolution?” vol. 274, no. 1617, pp. 1553–1560, Jun. 2007, doi: 10.1098/rspb.2007.0281.
[11] J. J. Soler, J. M. Aviles, M. Soler, A. P. Moller, “Evolution of host egg mimicry in a brood parasite, the great spotted cuckoo,” vol. 79, no. 4, pp. 551–563, Aug. 2003, doi: 10.1046/j.1095- 8312.2003.00209.x.
[12] K. Schulze-Hagen, B.G. Stokke, T. R. Birkhead, “Reproductive biology of the European Cuckoo Cuculus canorus: early insights, persistent errors and the acquisition of knowledge,” J Ornithol 150, 1–16 (2009). https://doi.org/10.1007/s10336- 008-0340-8
[13] S. I. Rothstein, “Mechanisms of avian egg-recognition: Do birds know their own eggs?” vol. 23, pp. 268–278, May 1975, doi: 10.1016/0003-3472(75)90075-5.
[14] O. Krüger and N. B. Davies, “The evolution of egg size in the
Ella Speers - BSc, Marine Science, Biological Sciences
Ella is a third-year student majoring in Marine Science and Biological Science. She is extremely passionate about counteractive measures against climate change, and marine ecology. Her area of interest is in restoration and conservation of marine habitats. She cannot wait to make a difference in fragile ecosystems.