The Gemara responds by pointing to the bat ha-ya’anah, one of the birds in the list of unclean birds (see Vayikra 11:16). Literally, the word bat seems to indicate that this is the daughter of the ya’anah and if the ya’anah does have a daughter – will the daughter have a different status than its mother? The Gemara concludes that it must mean that the egg of an unclean bird is not kosher.
It would appear that Hizkiah’s question is superfluous, given the general Talmudic principle ha-yotzeh min ha-tameh, tameh – that anything produced by an unclean animal is also unclean.
The Ba’al Halakhot Gedolot argues that the true issue at hand is how we know that the egg of a clean bird is permitted by the Torah. That is to say, a living bird is forbidden to eat until it is slaughtered, so we may have thought that the eggs it produces are forbidden as well. The passage about the bat ha-ya’anah teaches that only eggs of unclean birds are forbidden.
Other rishonim disagree with this approach. The R”i suggests that we know that the eggs of a kosher bird are permitted from the law of shilu’ah ha-ken, where the Torah permits taking the eggs of a bird if you first chase the mother away (see 22:6-7). It is possible that we may have thought that since the Torah permits the use of such eggs – their status as the product of a non-kosher (i.e. a living) animal notwithstanding – we may have thought that all eggs would be permissible. This is the foundation for Hizkiah’s question and the Gemara’s response.
The bat ha-ya’anah is usually identified as the ostrich, struthio camelus, which is its name in modern Hebrew. Some researchers suggest that it is a nocturnal bird of prey, the Eurasian Eagle-owl, or bubo bubo.