 A nestling begins to give the ' chip ' call soon after it emerges from the egg , and it calls frequently for the first two thirds of the nestling period and constantly for the last third . The call does not vary in the presence of an adult at the nest , so it seems likely that the call is not directed at the adult bird . The nestling becomes silent when an adult gives an alarm call , and makes a <unk> sound when the adult delivers food . The <unk> 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 . <unk> , a juvenile vocalization comprising elements of various calls , begins to be uttered when the fledgling is around thirty days old . 
