A nestling begin to give the ' chip ' call soon after it emerge from the egg , and it call frequently for the first two thirds of the nestling period and constantly for the last third . The call do not vary in the presence of an adult at the nest , so it seem likely that the call is not directed at the adult bird . The nestling become silent when an adult give an alarm call , and make a squealing sound when the adult deliver food . The begging call of a fledgling is similar to the call of the nestling , but significantly louder and covering a greater frequency range ( which may make it more directional ) . The rate of calling , on average , is 85 to 100 calls in a minute , and in open scrub the call can be heard up to a kilometre away . Subsong , a juvenile vocalization comprising elements of various calls , begin to be uttered when the fledgling is around thirty days old .