chiffchaff
Concise Oxford English Dictionary © 2008 Oxford University Press:
chiffchaff/ˈtʃɪftʃaf/
▶noun a common warbler with drab plumage and a repetitive call. [Phylloscopus collybita.]
– origin C18: imitative of its call.
'chiffchaff' also found in these Oxford entries:

