Jacobin Cuckoo 

One of the aspects of nature that I love is that I am always learning something new… As are the scientists. In this incredible world that we inhabit, there are things that take place that positively beggar belief! So, as per Alice’s Adventures in Wonderland, down the rabbit hole we go… This week we explore birds, and their eggs. Or more specifically, cuckoos – and in particular the Jacobin Cuckoo.

The cuckoo in this week’s feature is black and white and was named after the Jacobins – a Dominican Order of monks in the Middle Ages in France who used to wear habits that were black and white. There are two colour morphs. The first is with white underparts, and the second is completely black except for the white wing bar. 

Jacobin cuckoos occur south of the Sahara in Africa and south of the Himalayas in India. The Jacobin cuckoo is only found in our part of the world during the summer months (known as a “summer resident”). It is an “Intra-African breeding migrant”, with complex movements across its distribution range. It normally arrives in southern Africa (from further north) in October to breed, and usually leaves by April, although it sometimes stays for the rest of the year. They are also one of the most iconic migrants of the Indian subcontinent, and their arrival in northern India is considered to herald the first monsoon rains. Although resident in southern India, the central and northern populations migrate over southern Arabia to east Africa for the winter. Its chosen habitat is Acacia and mixed savannah woodlands, and also valley bushveld and coastal forest. 

Its favourite food is spiny or hairy caterpillars, as well as the smooth caterpillar of the well-known army worm. These are passed back and forth through their bills, and also swung side to side until they are flat, and all the caterpillar’s gut content is cleaned out. Only then are they swallowed. They have also been known to eat termite alates, as well as the eggs of their host species… Now let’s explore their breeding habits because this is where it gets interesting!!

These birds (and most cuckoos) are known as brood parasites. Which means the female will lay her eggs in the nests of other birds, who will raise her chicks. According to Roberts Birds of Southern Africa, the major hosts include the Dark-capped Bulbul, Cape Bulbul, Common Fiscal (shrike), Sombre Greenbul, and African Red-eyed Bulbul. Eggs have also been found in the nests of the Speckled Mousebird, Fork-Tailed Drongo, African Paradise-Flycatcher, Southern Tchagra, Southern Boubou, Bokmakierie, Fiscal Flycatcher, Terrestrial brownbul, Cape White-eye, Chestnut-vented Tit-babbler, Cape Wagtail and Golden-breasted Bunting.

The eggs are laid in the host’s nest, usually between 07h00 and 09h00. The female perches quietly near the chosen nest, while the male perches conspicuously and starts to call, eventually moving closer until right above the nest, which causes the host birds to attack. At that point, the female quickly nips in and lays her egg (a process which takes about ten seconds). The host birds are none the wiser. The cuckoo chick hatches after 11 days (always before the host’s chicks), and its nest mates seldom survive. This cuckoo usually doesn’t evict its nest mates like many other cuckoos do, but by claiming most of the food brought back to the nest it still causes the other chicks to starve. The smaller host chicks are often trampled by the much larger cuckoo chick. This species usually lays one egg per host nest (less often two or three, rarely up to seven). It is said that they are able to lay up to 25 eggs in a breeding season!

The eggs are glossy white in South Africa, and mostly greenish blue in India, Sri Lanka and some regions of Africa. It’s interesting that the Jacobin cuckoo’s eggs are pure white, which contrasts rather obviously with the host’s eggs. It is thus strange that the hosts accept the eggs, particularly as so many cuckoos perfectly match their eggs with those of their hosts. Two cuckoos may also lay their eggs in the same host nest, resulting in more cuckoo eggs than host eggs!

Oliver Kruger asks a question in his study (see link in info block): “But what about cases where the cuckoo egg is not mimetic and where the host does not act against it? Classically, such apparently non-adaptive behaviour is put down to evolutionary lag – given enough time, egg mimicry and parasite avoidance strategies will evolve.”

Many of the Cuckoo species differ hugely on breeding behaviour. This quote was taken out of “A Guide to the Nests & Eggs of Southern African Birds” – a delightful book to buy if you’re interested in birds. “The southern African species do not build nests or raise their own young, but instead parasitize other birds. Beyond this shared behaviour, they diverge widely in terms of breeding habits. Some are monogamous, others promiscuous. Some are host-specific, others parasitize a range of hosts. Some lay eggs that closely match those of their hosts, others lay eggs that are conspicuously different. Some lay a single egg per host nest, others lay several. Some remove host eggs when they lay, others don’t. In some, the cuckoo nestling evicts the host’s egg, in others it doesn’t. All southern African cuckoos do, however, share the following: their incubation periods are shorter than those of their hosts, and they lay their eggs in the hosts’ nests while the hosts are laying their own clutch. The cuckoo eggs therefore hatch ahead of their hosts’, usually resulting in the demise of the host’s brood.”

For me, the most interesting aspect of these behaviours is when individual female cuckoos have evolved to the extent where they lay eggs of different colours to match those of their hosts (egg evolutionary races). Some will lay dark eggs in dark nests, so that they are almost invisible to the host. Finally, they are able to lay an “average” egg – an egg that is not a perfect match to any host egg, but a pretty good match for many of the host eggs.

Absolutely fascinating!

Jacqui Ikin & The Cross Country Team

INFO BLOCK:

Egg colours make cuckoos masters of disguise (great video in this article):
https://theconversation.com/egg-colours-make-cuckoos-masters-of-disguise-34217

Brood parasitism selects for no defence in a cuckoo host. By Oliver Kruger.
https://www.researchgate.net/publication/49805590_Brood_parasitism_selects_for_no_defence_in_a_cuckoo_host

Egg-spot matching in common cuckoo parasitism of the oriental reed warbler: effects of host nest availability and egg rejection:
https://avianres.biomedcentral.com/articles/10.1186/s40657-016-0057-y

Cuckoos use host egg number to choose host nests for parasitism:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7341929/\