Vocal development of nestlings of Canary lsland Chiffchaffs leads from short simple elements to complex calls that show a large number of frequency bands. Calls do not develop at a uniform rate but change their duration, frequency and shape more rapidly during the last 4-6 days before fledging than before. Duration increases during development. Until 8 days before fledging the duration lengthens from 4 ms to 22 ms. Three days before fledging calls last 22-27 ms, increasing to 133-160 ms at fledging day. The frequency used by nestlings is higher than that of adult singing males. Maximum frequency is around 8 kHz but minimum frequency increases from 5 days before to fledging. Constant maximum frequency and rising minimum frequency leads to a smaller frequency range. The structure of calls also changes during development. Starting with simple hooked notes with rapid frequency changes until 8 days before fledging, they lead to element types that are modulated in frequency during the last part of voice development until fledging. Rising minimum frequency will be discussed as a protection against Sparrowhawks Accipiter nisus, the only avian predator on the islands. The ability to locate high frequencies is decreases with rising frequency [thanks to Hessische Landestrraduiertenforderung and Deutscher Akademischer Austauschdienst for support].