The Australian Museum says:
Most Koels migrate from Australia to New Guinea and probably eastern Indonesia and even further north, but some remain in northern Australia. During breeding season, they are found in northern and eastern Australia, south to about Nowra, New South Wales, although occasional birds are encountered further south. … Common Koels are found in tall forests and are common in suburban areas.
In late September and early October each year, Common Koels arrive in Australia from their northern winter homes to breed. The Koels leave southern Australia in about March.
Birds in Backyards notes that:
The male Common Koel advertises its presence by a loud ascending whistle or ‘koo-el’, monotonously repeated; the call of the female is a repetitive ‘keek-keek-keek-keek’. Males often call throughout the day and well into the night.
Both of them are right on all points I’ve quoted. The Koel in my photo, taken two days ago, is one of the first I’ve seen this season; he’s a male (females are camouflaged in mottled stripes of grey-brown – here is one from a couple of years ago) and he has been calling constantly since he turned up. We sometimes call them Cooee-birds because of their call but they are more widely known as Storm-birds or Rain-birds because they seem to call even more often in the lead-up to rain.
The true koels, Eudynamys, are a genus of cuckoos from Asia, Australia and the Pacific. They are large sexually dimorphic [i.e. male and female look different] cuckoos which eat fruits and insects and have loud distinctive calls. They are brood parasites, laying their eggs in the nests of other species.
In Australia, their hosts are mainly large honeyeaters (especially Noisy Friarbirds and Red Wattlebirds) [maybe Friarbirds and Blue-faced Honeyeaters too?]. Unlike in other parasitic cuckoos, the young do not attempt to kill the host chicks. This trait is shared with the Channel-billed Cuckoo [see An Unexpected Visitor], which – as in the Pacific Koel – are largely frugivorous as adults.
The only area in which there is disagreement between my sources (and I can add Birdway and Slater’s Field Guide to the list) is the correct name for our visitor. Most say “Common Koel” and “Eudynamys scolopacea” but the common name varies wildly – Australian/Eastern/Asian/Pacific Koel – while the Latin names are just as bad – Eudynamys cyanocephalus/orientalis/scolopacea. I consulted my Friendly Local Expert and he was so thorough and lucid that I will quote his response directly:
You’ve unearthed a messy can of worms in the taxonomy of Koels, something that has been in a state of flux since 1766 with major perturbations in 1802, 1903, 1940, 1997, 2005, 2008 and 2011 with the total number of species varying from one to four. … The current treatment is to recognise three species and consider the Australian bird the same species as the ones that occur in New Guinea and Indonesia and call it the Eastern Koel, Eudynamys orientalis, but treat the ones that occur in India, China, Indochina and Malaysia as the Asian Koel, Eudynamys scolopaceus. (The third species, the Black-billed Koel E. melanorhynchus occurs only in Sulawesi and Sula.)
By “current treatment” I mean there is general agreement since 2011 among BirdLife Australia, BirdLife International, the IOC and Clements. So it will remain until someone does some more DNA work and upsets the apple cart again. The irony with all this is that Lynnaeus named it E. orientalis in 1766, so I hope he’s feeling smug.
The BirdLife Australia taxonomic list, published in June, shows that the Eastern Koel in Australia belongs to two races, the South-east Eastern Koel (Eudynamys orientalis cyanocephalus) and the Torresian Eastern Koel (Eudynamys orientalis subcyanocephala). The one we get here is the Torresian.
So there we are: the handsome, garrulous gentleman in my photo is a Torresian Eastern Koel, Eudynamys orientalis subcyanocephala.