A chorus of cuckoos
Spring in Hong Kong is announced by a veritable cacophony of cuckoos. First, as early as late February, come the resident Asian Koels (噪鹃, "noisy cuckoos") and hooting Greater Coucals (褐翅鴉鵑) whose songs inspire their onomatopoeic names. In April the Plaintive Cuckoos (八聲杜鵑) join in with their falling, accelerating 8-syllable song. By early May, the Large Hawk-Cuckoos (鷹鵑) with their feverishly repeated 3-syllable song are heard throughout our woodlands but rarely seen except at Mai Po, where they are joined by the Indian Cuckoos (四聲杜鵑) with their 4-syllable song, sometimes transcribed as 'one more bottle'. Adding to the cuckoo chorus are other birds which mimic their songs, notably the Oriental Magpie-Robin which does a passable imitation of the Plaintive Cuckoo.
A surprise in recent weeks has been the addition of two 'Common' or Eurasian Cuckoos, perching and occasionally singing around the police post at Mai Po. They appear to be a male and a female, presumably a pair on northbound passage, with the female showing a rufous throat. In much of Europe, the two-syllable song of these birds marks the arrival of spring and gives rise to the onomatopoeic name "cuckoo". It makes its way into Beethoven's 'Pastoral' Symphony and even inspired a piece of its own, Delius's 'On hearing the first Cuckoo in Spring'. Curiously, late in the breeding season the song shifts from a falling major 3rd to a falling major 6th, giving rise to the rhyme "The Cuckoo comes in April, sings his song in May, changes his tune in the month of June" and to further musical possibilities. In his 1st Symphony Mahler even transforms the interval into a 4th, taking a certain poetic license in order to echo the primeval falling 4th of his opening bars.*
*This is the account repeated by Mahler specialists such as conductor Michael Tilson-Thomas and biographer Henri-Louis de la Grange, who implies that it originated with Mahler himself. But the apparent discrepancy between nature and art is not so clear. In May 2024 at Fowlmere in Cambridgeshire, one cuckoo was indeed producing a falling 4th, while others were producing either a minor or a major 3rd. It appears that the interval varies between individuals and gradually widens before reaching a major 6th in June. So there is a basis in nature for Mahler's treatment of the cuckoo's song. A recording is here:
https://ebird.org/checklist/S174213739