Common chiffchaff one of the first avian signs that spring has returned. Its call is a "hweet", less disyllabic than the "hooeet" of the willow warbler or "hu-it" of the western Bonelli's warbler. The song differs from that of the Iberian chiffchaff, which has a shorter . However, mixed singers occur in the hybridisation zone and elsewhere, and can be difficult to allocate to species. When not singing, the common chiffchaff can be difficult to distinguish from other leaf warblers with greenish upperparts and whitish underparts, particularly the willow warbler. However, that species has a longer primary projection, a sleeker, brighter appearance
Hawking (birds) nighthawk does not). When different kinds of birds have the same adaptations, such similarities are not necessaily indicative of any familial relationship between bird species. Rather, they are the result of convergent evolution. Consider, for example, the marked resemblance in body size, shape, and coloration between flycatchers of several families, though these species are not closely related: the Asian brown flycatcher (of the Muscicapidae or Old World flycatcher family), Acadian flycatcher (of the Tyrannidae or tyrant flycatcher family) of the New World, and slaty monarch (of the Monarchidae or monarch flycatcher family), endemic to Fiji. All three use flycatching to