Sumer is icumen in/ Lhude sing cuccu

[Translation: Summer is approaching, loud sings the cuckoo — a 13th-century English song]

Just days before the onset of austral spring on September 1st, I heard the first cuckoo call of the season. It was a male Pacific Koel, an elegant bird with glossy black feathers and striking ruby eyes, known for its haunting call that travels far, particularly at night. These migratory birds winter in New Guinea before returning to Australia with the arrival of warmer weather and monsoon rains. Often referred to as Stormbirds, their calls herald the onset of the Wet Season.

The cuckoo mentioned in “Sumer is icumen in” refers to the Common Cuckoo found across Europe and Asia. Naturalists have long documented the arrival of this bird, a practice that eventually made its way into the letters section of The Times.

William Marwick (1789) observed that cuckoos would typically appear in southern England in late April and leave by mid-September. Notably, on February 4, 1913, naturalist Richard Lydekker wrote to The Times to highlight an unusually early sighting of a cuckoo in Harpenden, only to later reveal that the sound came from a bricklayer skillfully mimicking the bird.

The phenomenon of seasonal migration, which involves traveling between breeding and wintering grounds, is intricate and primarily driven by food availability. Many species, including the Common Cuckoo, cross the equator to experience continuous summer. Some, like the Short-tailed Shearwaters, migrate from the northern Pacific to southern Australia and back again as the southern winter approaches. Arctic Terns take the longest journeys, traversing between the Arctic and Antarctic circles.

Conversely, some birds are less adventurous. For example, Steller’s Eider migrates from northern Alaska and Russia to the Kuril and Aleutian Islands, sometimes reaching the Baltic Sea. Migration is a taxing endeavor, so it’s sensible for these birds to avoid unnecessary travel.

Historically, humans have been aware of the Common Cuckoo's migratory behavior. Aristotle noted that some believed the cuckoo transformed into a Sparrowhawk, but he dismissed this notion:

>The hawk has crooked talons, which the cuckoo does not possess; moreover, the cuckoo does not resemble the hawk in head shape. In fact, both in head and claws, it resembles a pigeon more closely. In terms of color, it may look like the hawk, but the markings differ; the hawk has stripes, while the cuckoo is mottled. Size and flight patterns may align with the smallest hawk species, which typically disappears around the time the cuckoo reappears, though both have been observed together. Cuckoos have been known to fall prey to hawks, but this predation does not occur among members of the same species.

Aristotle also noted the cuckoo's peculiar behavior of laying its eggs in the nests of other birds.

Cuckoos tend to be elusive, often heard rather than seen. Pacific Koels typically hide among dense foliage, where their male's dark feathers and the female's brown and white plumage help them blend in. However, their presence is often betrayed by other bird species. In tropical Queensland, Hornbill Friarbirds, Australasian Figbirds, and Magpie-larks, which can be parasitized by Koels, will aggressively mob them. Hornbill Friarbirds are particularly vocal and will rally other birds to assist in driving away the Koel. Smaller birds, like Willie Wagtails, may join in, enjoying the commotion despite not being in danger.

No one appreciates a cuckoo in their nest.

While the Pacific Koel was the first cuckoo I heard, the first I actually saw was the Pheasant Coucal.

As the second-largest cuckoo species in Australia, surpassed only by the massive Channel-billed Cuckoo, Pheasant Coucals have a somewhat scruffy appearance. For much of the year, they hide in the underbrush, relying on their mottled brown feathers for concealment. However, during the breeding season, males showcase their vibrant black feathers, contrasting with the chestnut hues of their wings and tail. While still somewhat clumsy, a male in breeding plumage exhibits a certain flair.

When it comes to cuckoo behavior, Coucals are quite different. They are not migratory (due to their poor flying skills) and raise their own chicks, with males shouldering most of the parental duties. This independence allows them to enjoy the sun without worrying about the ire of Friarbirds. I first spotted a Pheasant Coucal lying in the garden, fluffed up and basking in the sun, seemingly unbothered by anything around (except for me).

Pliny the Elder suggested that cuckoos evolved to be nest parasites because other birds shunned them. He inverted the cause and effect, claiming that such attacks led cuckoos to believe they could not raise their own young, thus relying on other birds for this purpose to ensure their survival.

In De Natura Animalium, the Roman author Aelian noted that cuckoos lay their eggs in nests of birds with eggs that resemble their own.

>… si vero ova intus sunt, iis sua admiscet. Quando autem multa illarum ova adsunt, ea ejecta perdit; suaque, quae ob similitudinem dignosci nec deprehendi possunt, relinquit.

>[… but if the eggs are inside, he will mix them with his own. But when many of these eggs are present, they are thrown out and perish; and his own, which cannot be recognized or detected because of their resemblance, he leaves behind.]

Sixteen centuries later, Eduard Baldamus and Alfred Newton studied this behavior in the context of evolution.

>Without attributing any remarkable intelligence to her, it seems reasonable that a Cuckoo, which had successfully placed her egg in a Reed-Wren or a Titlark's nest, would continue this practice when she had another egg to lay, passing this behavior down through generations. Such habits are likely to become hereditary, so that the offspring of a Cuckoo that consistently uses a Reed-Wren's, Titlark's, or Wagtail's nest would follow in their mother’s footsteps. Furthermore, while there may be some variations in the eggs of different individuals of the same species, there is often a strong familial resemblance among the eggs laid by the same individual, even across many years. Thus, it stands to reason that the eggs of the daughter would resemble those of her mother. This supposition, combined with the idea that the Cuckoo’s habit of using the same species’ nests could also be hereditary, suggests that the principles of "Natural Selection" could explain the patterns noted by Aelian over time.

Newton coined the term gentes (singular: gens) to describe these host-specific lineages. For instance, a Cuckoo that specializes in laying eggs in Pied Wagtail nests belongs to a different gens than one that parasitizes Reed Warblers or Meadow Pipits.

These strategies depend heavily on the behavior of host birds — specifically, which hosts will incubate and rear the cuckoo’s young. With over 50 species of parasitic cuckoos and countless potential hosts, the combinations are virtually limitless. Typically, where hosts are more discerning, cuckoo eggs closely match those of their hosts; otherwise, a more random approach may occur.

In Africa, Fork-tailed Drongos excel at detecting cuckoo eggs, particularly those of the African Cuckoo, which closely resemble their own. They can identify and reject the impostors with 93.7% accuracy.

The Horsfield’s Bronze-cuckoo, found in Australia and Indonesia, lays its eggs in the nests of 17 different bird species. The eggs of the Bronze-cuckoo are a compromise, resembling all potential host species yet identical to none.

Even the master mimic, the Common Cuckoo, does not always match its eggs perfectly. The eggs laid in Dunnock nests are noticeably different — speckled as opposed to the Dunnock’s plain blue — yet the Dunnocks will still rear the Cuckoo chicks.

>In the meantime, the female bird, sitting on her nest, is rearing a supposititious and spurious progeny; while the young cuckoo, which is naturally craving and greedy, snatches away all the food from the other young ones, and by so doing grows plump and sleek, and quite gains the affections of his foster-mother; who takes a great pleasure in his fine appearance, and is quite surprised that she has become the mother of so handsome an offspring. (Pliny the Elder)

In contrast, Pheasant Coucals go about their lives without any deceit. They lay their large white eggs in nests that are either constructed on the ground or situated low in dense vegetation, raising their scruffy chicks with care and perhaps even pride.

Meanwhile, Hornbill Friarbirds and their cohorts pursue Koels, giving them a hard time whenever they venture above the leafy cover. The calls of Koels resonate throughout the summer, both day and night, gradually diminishing as the season concludes and they prepare for their migration north.

As the Koels depart for warmer territories, the Common Cuckoo will also be journeying northward, crossing the equator from southern Africa to Europe and Asia.

Cuccu cuccu/Wel singes þu cuccu [Cuckoo, cuckoo/Well sings you, cuckoo]

Species mentioned: - Pacific Koel (Eudynamys orientalis) - Common Cuckoo (Cuculus canorus) - Short-tailed Shearwater (Ardenna tenuirostris) - Arctic Tern (Sterna paradisaea) - Steller’s Eider (Polysticta stelleri) - Sparrowhawk (Accipiter nisus) - Hornbill Friarbird (Philemon yorki) - Australasian Figbird (Sphecotheres vieilloti) - Magpie-lark (Grallina cyanoleuca) - Willie Wagtail (Rhipidura leucophrys) - Pheasant Coucal (Centropus phasianinus) - Channel-billed Cuckoo (Scythrops novaehollandiae) - Pied Wagtail (Motacilla alba yarrellii) - Common Reed Warbler (Acrocephalus scirpaceus) - Meadow Pipit (Anthus pratensis) - Fork-tailed Drongo (Dicrurus adsimilis) - African Cuckoo (Cuculus gularis) - Horsfield’s Bronze-cuckoo (Chrysococcyx basalis) - Dunnock (Prunella modularis)