母鸡 (mǔ jī) means “hen” | MotiTan Chinese Dictionary

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给母鸡接种疫苗可以防止疾病传播。gěi mǔ jī jiē zhòng yì miáo ké yǐ fáng zhǐ jí bìng chuán bō

Vaccinating hens can prevent the spread of disease.

The domestic chicken viewed as a mature female kept for eggs or breeding.

鸡jī: A chicken in general, used when the bird's sex is not specified.
公鸡gōng jī: A male domestic chicken that gives a distinctive crow.

家禽中的成年雌鸡，常用于表示产蛋或供饲养的雌性个体。jiā qín zhōng de chéng nián cí jī cháng yòng yú biǎo shì chǎn dàn huò gōng sì yǎng de cí xìng gè tǐ

母 (mǔ, "mother or female") + 鸡 (jī, "chicken") combines a female marker with the name of the domestic animal to specify an adult female chicken.

In everyday counting, 只 (zhī, "measure word for animals") is the usual classifier for this noun, while 个 (gè, "general measure word") is not the natural choice.