noun
A small whistle used to produce a sound, often for signaling or attracting attention.
登山のときは、呼び子を持っていくと安心です。
When mountain climbing, it's reassuring to bring a whistle.
笛 is a general term for flutes and whistles, while 呼び子 specifically refers to a small signaling whistle.
Compound of 呼び (yobi, 'calling', the continuative form of 呼ぶ yobu) and 子 (ko, 'small thing'). Literally 'calling device'.