Some birds have a simple calls but these often hold an iconic position in our collective memory. Let's listen to these two-note troubadours. 

Acknoweldgements

• Great Tit https://xeno-canto.org/788731 
• Coal Tit https://xeno-canto.org/717705 
• Blue Tit https://xeno-canto.org/1070960 
• Common chiffchaff https://xeno-canto.org/637792 
• Willow warbler https://xeno-canto.org/915783
• Cuckoo https://xeno-canto.org/673118 
• Corncrake https://xeno-canto.org/810279 
• Phil Barnett, for his discussion on thsi topic
• Róisín Costello, Of corncrakes and Irish. https://irishpages.org/down-to-earth/?v=25400724d737
• https://www.corncrakelife.ie/
• Ręk, P. (2013). Acoustic location of conspecifics in a nocturnal bird: The corncrake Crex crex. Acta Ethologica, 17(1), 31–35. https://doi.org/10.1007/s10211-013-0155-3