sentinel

She said it is important to remember that in villages, there will always be an increasing number of sentinel birds between vaccination campaigns as chicks are constantly hatching..她说，在农村，在接种疫苗的战役之间总是存在数量不断增长的"哨兵"禽类，因为小鸡会不断地孵化出来。 意识到这一点是很重要的。

Moreover, at the gateway of every reading lesson stood sentinel an array of words, with separated syllables, and forbidding accent marks like fixed bayonets, barring the way to the infant mind.. 在每一课文的入口，都排列着一队音节分开的生字的哨兵，禁止通行的重音符号就像一把把瞄准的刺刀，阻挡了小孩的心的进入通道。

stand sentinel,站岗，放哨，守卫