The extraordinary breeding behaviour of the Black Coucal Centropus grillii has piqued the interest of ornithologists for decades, so much so that the species is now regarded as one of the most unusual birds in the world. Of the 28 coucal species found globally, it is the only one known to be polyandrous. The southern African population, moreover, is the only one that is migratory.
Polyandry is rare among the world’s birds and has been recorded in only about one per cent of species. In this role-reversal mating system, after laying the eggs the female plays no further part in raising the young. What makes the Black Coucal even more remarkable is the fact that its offspring are altricial (hatched in an undeveloped state and requiring care and feeding by the parents), which is almost unknown in polyandrous species. In general they rear precocial chicks, which are far easier to manage as, almost from day one, they are able to accompany the males to feeding sites where they are fed and protected by the male. In the subregion, the species best known to practise this mating system is the African Jacana, but there are others, such as the Greater Painted-snipe and the three buttonquail species.
Among polyandrous species, however, the Black Coucal differs in that the males not only are responsible for nest building, incubation and rearing the offspring, but they also have the task of provisioning and protecting altricial chicks for the two-week nestling period. Once the chicks are three or four days old, the male leaves them alone and unattended in the nest for at least 95 per cent of the day while he searches for food as far as half a kilometre away. During this time, he maintains an exceptionally high rate of prey delivery, which can only be achieved at a site where there is an abundant supply of invertebrate prey.
The pressure on the male during the breeding cycle is huge compared to that of the female, which never goes near the nest after having laid the eggs. During summer, the female spends most of her time within the territory soliciting for more potential mates. Amazingly, should the parenting male succumb to predation during the nestling period, the female would continue looking for a mate nearby, unaware that her offspring are starving to death.
With very little suitable damp lowland grassland remaining in South Africa, the Black Coucal is now one of the rarest breeding birds in the country, with an estimated population of only about 20 breeding groups remaining. Outside national parks, the population is presently so small that the chances of a female being able to attract additional males to a territory are slim. The two core breeding areas for this species are the grasslands surrounding Lake St Lucia and, to a lesser extent, selected areas in the Kruger National Park. The nearby Mozambican coastal grasslands, dominated by cottonwool grass Imperata cylindrica, are unsuitable as a breeding habitat for this species as it needs thick, knee- to waist-high grassland and shrubbery, which serve as a protective refuge from possible raptor predation.
The first Black Coucal found in southern Africa was collected in the 1840s near the Ohlanga River just north of Durban. Although nowhere common, it would have been thinly distributed in suitable damp lowland grassland throughout the coastal belt of northern KwaZulu-Natal. With the almost complete loss of this habitat in KwaZulu-Natal during the subsequent 200 years, the breeding population outside protected areas now stands at a mere handful of birds. The overall population in South Africa, including those in protected national parks, is estimated to be in the region of approximately 20 groups or ‘pairs’.
During the summer of 2018/19, a pair of Black Coucals was found to be breeding at Mtunzini, which is situated on the KwaZulu-Natal coast about 150 kilometres south of Lake St Lucia. The birds occupied an approximately 20-hectare block of moist lowland grassland that is traversed by Phragmites australis reedbeds and lies only four to five metres above sea level. Until as recently as 10 years ago the area was agricultural land used for sugarcane cropping, but it is now managed by Ezemvelo KZN Wildlife. The abandoned sugarcane fields have reverted to waist-high grassland where the dominant grass species, common finger grass Digitaria eriantha, is interspersed with shrubs such as velvety indigo Indigofera tristis and silver tephrosia Tephrosia purpurea. They now support such a healthy invertebrate population that in the summer of 2019/20 these migrant coucals had sufficient confidence to attempt three broods, with only weeks between the offspring fledging and the next clutch being laid.
Shortly after arriving from migration in early November 2019, the male began nest building. The first brood of chicks fledged on 14 December, little more than a month after the male had arrived. The nest, an oval ball with a side entrance built of grass and sparsely lined with green leaves, is usually placed just above the ground at the base of a grass tuft. Leaf-carrying is usually the tell-tale indication that nest building is underway.
The second clutch of three eggs was laid four weeks after the three offspring from the first nest had fledged. Then to our amazement, on 1 March 2020, little more than two weeks after the chicks had fledged from nest two on 15 February, the same male was found attending a third nest with a one-day-old chick and an egg. The incubation period for these birds is 14 days, which meant that the male had started building the third nest while he was still feeding chicks in the second nest. He began incubating at nest three either the day before or perhaps the day after the last nestling in the second nest had fledged. It was remarkable behaviour for so late in the breeding season.
The female Black Coucal is capable of laying a number of clutches in a season, especially when two or three males are present. Individual males occupy separate nests, spaced within the female-dominated territory. The single male at the Mtunzini grassland site in the 2019/20 summer was able to rear three broods, successfully fledging six offspring. The first two nests had clutches of three, but one egg in the second nest turned out to be addled. A clutch of two was laid in the third nest, but again, one was added.
The polyandrous mating system with single adult participation is likely to succeed only in subtropical climates, where the brooding of young chicks can be kept to a minimum. After the first few days, the male seldom brooded the chicks for long, even in the early morning when temperatures were at their lowest. This gave him more time to provision the chicks with prey scouted from the adjacent grassland and reedbeds, a task that began at sunrise and continued almost until sunset.
Midsummer breeding coincides with a flush of half-grown nymph grasshoppers, which was the food source most utilised by the coucals, comprising 60 per cent of the 176 identified prey items delivered to the nests. Katydids and praying mantids were the next most common items. When the nestlings were still young, the male-targeted green nymphs as prey items. These soft nymph grasshoppers are usually fed whole to nestlings, although the heads are occasionally removed. In the case of larger, mature grasshoppers being brought as prey, all the extremities, including the wings, are stripped off.
The most surprising item fed to the chicks was crab, as the hard carapace and remaining legs would have been difficult for a nestling to swallow. Another surprising prey species was a dragonfly, a female vagrant emperor Anax ephippiger, not because it was uncommon in the grasslands or an unsuitable prey item, but because of its speed and agility. The dragonfly was caught at 08h00, two and a half hours after sunrise on a predominantly overcast morning in December, so it was probably still resting on a grass stem when it was snapped up. Of all the identified prey items offered to the chicks, crabs were delivered only twice and dragonfly once.
One of the most noteworthy differences in prey composition between the two successive nesting attempts was the high number of stink bugs caught as prey in December (14 per cent), whereas no bugs or twig writers were fed to the two nestlings during February. Perhaps this indicated an absence of these species in late summer as opposed to early summer in that particular habitat? Another surprise in prey composition was the lack of beetles and the low number of caterpillars. Beetles were seen in the vicinity, but none was brought in as prey.
Surprisingly, the only vertebrate prey was reed and water-lily frogs, even though lizards, skinks and small snakes were present in the adjacent grasslands. A small brown house snake Boaedon capensis was seen approximately 50 metres from the February nest. Frogs comprised only 10 per cent of the prey and, interestingly, only water-associated species were identified. Painted reed frogs Hyperolius marmoratus and tinker reed frogs H. tuberilinguis were the most common frog prey species, while only two water-lily frogs H. pusillus were delivered. Grass frogs, in particular Ptychadena species, were occasionally heard calling in the vicinity, sometimes within metres of the second nest, but never noted as a prey item.
The high prey delivery rate recorded at the first nest (November/December) was astounding, a reflection of the arthropod richness of the surrounding grasslands. Ninety-three prey items were brought to the nest during a period of eight hours and five minutes, reflecting an average delivery rate by a single parent of one item every 5.2 minutes – an almost unequalled workload for any non-passerine. Deliveries began shortly after sunrise and continued until late afternoon. Also, post-midday deliveries continued at the same rate as those in the morning. Interestingly, pouring rain was no deterrent to the delivery rates; in fact, light rain seemed to enhance the male’s ability to catch prey.
When they are 14 days old, the nestlings fledge in the early morning. They remain flightless for approximately two weeks and, remarkably, the male-only feeds them on the day on which they leave the nest and then totally ignores them. The fledgelings have to survive on small insects they can scrounge from their almost waist-high grassy habitat. This is perhaps one of the most extraordinary features of this coucal’s breeding biology – the spiky, rodent-like fledgelings are left to find their own insects and fend for themselves while they are still unable to fly. One exception was the smaller chick in the second nest in February. It was small, feeble and dominated by its older and stronger sibling, which had fledged three days earlier. Unlike the other nestlings, this runt was occasionally fed by the male for almost a week after it had left the nest, by which time it had moved about 200 metres away.
Black Coucal offspring don’t emerge from the thick grass cover for about three weeks and when they do, they only show themselves above grass level in the early morning, when they perch on low bushes to sun themselves. The adults also sun themselves for 20 to 40 minutes each morning.
Black Coucals can be surprisingly difficult to locate, in spite of their bold black plumage contrasting against the open grassland habitats they occupy. During the hot summer months, when they are not searching for food, they spend much of the day shaded by large grass tufts. During the breeding season, while the females perch prominently and call for long periods in the hope of enlarging their harems, the males are usually hidden from sight, searching for food or incubating.
The larger and more dominant Burchell’s Coucals Centropus burchellii overlapped and occupied the same grassland territory, but they tended to remain more on the margins in sparsely spaced shrubs and small trees. When in close proximity, Burchell’s adults were intolerant of adult Black Coucals and frequently drove them away. Juveniles tended to be more forbearing; juvenile Black and Burchell’s were seen perched together in the same bush. These grassland coucal species are always alert to overflying raptors and never approach the nest when birds of prey are in the vicinity.
The long-term future for Black Coucals outside protected areas in South Africa does not look promising. While the two core breeding areas, either side of the Lake St Lucia system and in the central Kruger National Park, could possibly maintain viable breeding populations, elsewhere their population is now probably too low because breeding groups need two or more males to keep the population at sustainable levels. There is simply insufficient suitable damp grassland habitat available for this migratory species.
The female at the Mtunzini site solicited continuously throughout the summer months in her bid to attract another male to the territory. She continued through to late February when there was little hope of successfully rearing another brood prior to migration. Historically, females would have been able to attract more than one male to breeding sites. However, with the lack of suitable habitat this is now seldom recorded, so population recruitment for this polyandrous migratory species with an imbalanced sex ratio is at a minimum. It is likely to decline further until there are virtually no birds breeding outside the larger protected areas. At current population levels, this species now rates as one of the rarest breeding birds south of the Limpopo River.
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