那些卫兵是熟练的口哨手，他们用哨声进行远距离交流。

The guards were skilled ___whistlers, using their signals to communicate across long distances.

清晨散步时，我们听到树林里传来啸鹟鸟悦耳的鸣叫声。

During our morning walk, we heard the beautiful calls of ___whistlers in the woods.

暴风雨期间，无线电爱好者经常能听到奇怪的哨声信号。

During storms, radio enthusiasts often pick up mysterious ___whistlers in their receivers.