An evolutionary arms race and cultural traditions
Text and photographs Nicolette Forbes
The energy required for reproduction across the animal kingdom is considerable. In birds, this often includes production of eggs, courtship, building nests, incubating eggs, feeding the chicks, and protecting them from predators. Perhaps for this reason, about 1% of all bird species globally shortcut this process and utilise another strategy entirely. These are the birds known as brood parasites. Cuckoos are the most well-known of these but there are others.
Five different groups globally with three in South Africa
The interesting thing is that this strategy has developed more than once in birds. This is convergent evolution and is remarkable in that the feature arose independently in different lineages i.e. the species that have these similar features are not closely related but develop the trait completely independently through evolutionary history. Brood parasitism has done this in in five different groups of birds; the cuckoos already mentioned and also cowbirds, honeyguides, estrildid finches and a South American Duck. South Africa has three of these groups (honeyguides, viduid finches including the indigobirds, whydahs and cuckoo finch, and the cuckoos) which have this reproductive strategy in common but have significant differences in diet, habitats occupied and behaviours. Honeyguides are the only family of the three where the entire family uses this reproductive strategy.
All three honeyguide species (Greater, Scaly-throated and Lesser Honeyguides) and the Brown-backed (Wahlberg’s) Honeybird which are found in South Africa are obligate brood parasites. All four species may be seen in the Krantzkloof reserve areas and gardens of the upper Highway area. Most of them have a polygynous strategy in which the males’ mate with several females. This group lays their eggs in the nests of other cavity-nesters, with a wide range of hosts such as the bee-eaters, woodhoopoes, rollers, kingfishers, and barbets.
To ensure success of this breeding strategy, the females will lay only a single egg in the nest of each host to minimise the possibility of detection by the host. Additional protections are employed with some females puncturing the eggs of the host to ensure her chick receives all the food that the host birds deliver to the nest. To further cement the chances of survival, the honeyguide chick kills any remaining host nestlings with a sharp hook on its bill when it hatches and thus begins the parental responsibilities of the host. Different species of honeyguides are known to have a number of different hosts with the Greater Honeyguide (pictured below) known to have as many as 34 species of hosts.
A juvenile/female Greater Honeyguide
Some of the potential host or parent species:
Whydahs and Indigobirds
All the species in the family which contains the whydahs and indigobirds are brood parasites and they all, except one, use the estrildid finches such as the mannikins, firefinches, and waxbills as hosts. They are promiscuous, with males courting several females and females occasionally mating with several males. The female lays her eggs in the nest of a host after carefully watching nest construction. The brood parasite generally does not harm the eggs of their host, as the host pair are often able to raise some of their own chicks successfully as well as those of the parasite. Cuckoo Finches are the only species in this family that do not parasitise finches but rather use prinias and cisticolas as their hosts. They are also unusual in that the female removes the host eggs before laying her own.
The remarkable similarity in mouth markings between the chicks of the species in this group and their hosts has been interpreted as the result of the continuous arms race between parasite and host discussed in the last section of this article. Pin-tailed Whydah is the species representative of this group which may be found in the Upper Highway area. A fascinating bird with a bad rap which has people wanting to ‘deal with it’ because of its aggressive defence of its female harem at bird feeders. The whydah females once ready to lay an egg in the host nest will sneakily remove an egg from the host nest (it usually eats it) and replaces it with her own.
Pin-tailed Whydah and its host Common Waxbill
Village Indigobird with one of its common hosts, Red-billed Firefinch
Cuckoos are the group that is most well-known for this reproductive method and the most widely studied scientifically. However, not all the species in the cuckoo family (Cuculidae) are brood parasites. Coucals, for example, are in the same family as cuckoos but do not lay their eggs in other species’ nests. Perhaps, even more amazingly brood parasitism has evolved independently three times in this family and cuckoos parasitise a high number of hosts worldwide.
The more well-known and common cuckoo species in the Upper Highway area include the vocal and well-known Red-chested Cuckoo, and the green cuckoos; Klaas’ Cuckoo, African Emerald Cuckoo and Diederik Cuckoo. Red-chested cuckoos will target mostly Cape Robin-chats as their hosts if they are present but can use another 15 other species and in the Upper Highway region probably selects Red-capped Robin-chat most commonly. Diederik Cuckoo targets 24 known host species with a possible 12 others including Southern Red Bishops, widowbirds and sparrows while Klaas’ Cuckoo may choose from 18 host species including batises, sunbirds and the smaller warblers. The African Emerald Cuckoo is a bit more specific targeting Green-backed Camaropteras in our region and employs the tactic of evicting the host’s eggs from the nest.
The often elusive but vocal Red-chested Cuckoo
One of the hosts of the Red-chested Cuckoo and a common garden resident, the Red-capped Robin-chat
One of the three green cuckoos to be found in the Upper Highway area, the iridescent Klaas’ Cuckoo
An evolutionary arms race
While there are a few known tactics to allow a parasite to manipulate its hosts, the specialisation of the parasite and host species egg mimicry and recognition are the two most common strategies used to improve reproductive success. The development of these adaptations is a well-studied and accepted part of brood parasitism with continuous defences and changes involved chiefly with egg mimicry and nestling mimicry being termed the co-evolutionary “Arms Race”.
An intruder being able to successfully lay an egg in the nest of another species requires the progressive selection of host defence mechanisms. This in turn pushes for counter-adaptation in parasite species. This then progresses much like an ‘arms race’ does with host defence mechanisms needing to scale up or improve to keep outwitting the parasite and vice versa.
So over time new hosts rapidly evolve defences against brood parasites. Several studies have shown how host species learn to recognise and reject ‘foreign’ eggs when they are targeted by a brood parasite. It is equally well known that many brood parasites have evolved sophisticated egg matching to the host species, making it hard for hosts to detect these intruder eggs. Hosts counteract egg mimicry by either becoming extremely discriminating or by varying the pattern of their own eggs, making it harder for the parasite to match an individual’s pattern. Egg matching applies not only to colour and pattern, but also to egg size and shape and number. This is particularly critical in species such as honeyguides that target cavity-nesting hosts, for whom visual appearance might be less important in the darkness of tree holes or underground burrows.
This learning and adaptation may go beyond the egg stage to nestling mimicry and behaviour. If, for a host of reasons, egg recognition and rejection are too risky or costly to the host, then the recognition and rejection of nestlings may provide an effective alternative defence strategy. This ultimately results in plumage, mouth gape, and begging call mimicry. This is a much rarer strategy and only develops in situations where egg recognition methods fail.
Not so widely known is that some brood parasite chicks will also exhibit the incredible behaviour of emptying the nest of all the competition. This is well known in some cuckoo species. A chick working blindly and instinctively, apparently in response to any pressure on a specialised hollow patch on its back, gets underneath an egg or fellow nestling, backs up the side of the nest and tips it over the edge. Then it begs for food with a rapid call that mimics the whole brood of missing chicks.
In the same way that the mafia used to extract protection money there is evidence to suggest that this tactic is used in combination with the predominant strategies of mimicry and defence adaptations. This is an obvious result of the effect of brood parasitism on host species’ success and therefore rejecting the eggs or chicks of a parasite is an expected response.
Put another way because accepting foreign eggs and chicks directly reduces the number of offspring a host will produce; selection strongly favours hosts that reject them. However, studies in Spain (Great Spotted Cuckoo) and the USA (cowbirds) have shown that if a host bird throws out an intruder’s egg the brood parasite will take ‘revenge’ by destroying the entire nest sometimes repeatedly. Consequently, it is beneficial for hosts to be capable of learning to avoid this and to cooperate. This results in the hosts learning to accept a certain level of parasitism as long as they can raise their own offspring alongside the parasitic chicks. A forced co-operation learned over time and now termed the ‘Mafia Hypothesis’.
I am hoping by now this has opened up a whole range of questions by the reader and has inspired you to read more about the fascinating world of brood parasites. On a last note, there should be one question which was triggered by this article. If a single species of brood parasite is able to parasitise a whole range of species with different looking eggs (e.g. a Diederik Cuckoo can select a host from 24 species) how does an individual bird produce eggs that can mimic so many host eggs.
A fascinating answer to this is the development of host types called gentes. These different groupings within a species are called the gens of a species (plural gentes) and refer to the host-specific lineages of a brood parasite. This can be illustrated using the example of female Red-chested cuckoos who ‘learn’ to recognise the type of nest of her upbringing. So a Red-chested cuckoo female born in a Red-capped Robin Chat nest will remember the nest type of her origin and will find a Red-capped Robin Chat nest when she begins to breed. This means that different individuals within a species will remember different hosts. This is possible because the brood parasite remembers the nest type and passes on memories by cultural tradition from mother to daughter to granddaughter.
This characteristic runs through a number of different groups of brood parasites. Differentiation of host types or gentes in the Greater Honeyguide have been found to have arisen several million years ago in Africa. This is completely entrenched by the females of the species and is independent of the male representatives. The host specificity has evolved and is deeply entrenched through the action of females and their daughters selecting and adapting to particular host nest types across millions of generations while mating at random with respect to the host origin of their male partners.
It’s not a choice!
A final important point all of the behaviour or adaptations described in this article is that none of this should be seen as a choice or thought on the part of these birds. The responses and development of the specialisations and adaptations described are pre-programmed by its genetics and the pressures of natural selection.
About the author
Nicolette Forbes was born in Durban and is passionate about all things KZN and its environments. With an interest in all things living from a young age it was no surprise that her chosen career path ended with her becoming a professional biologist having studied biological sciences at the University of Natal, Durban (now University of KwaZulu-Natal). Studying was followed by a lecturing stint to both biology and medical students for nine years before leaving the university to put her knowledge into practice with an ecological consultancy specialising in coastal habitat assessments.
Birding has been a passion from her high school days and birdwatching, atlassing. photography and being in the bush are her favourite things. Currently the Chair of BirdLife Port Natal, the club covering the Greater Durban area, Nicolette has also through the non-profit EcoInfo Africa, partnered with Kloof Conservancy to run environmental courses focussed on birds, and these will continue once it is deemed safe to do so.