יורה דעה ט״וYoreh De'ah 15

Se’if 1א׳

אֶפְרוֹחַ כָּל זְמַן שֶׁלֹּא יָצָא לַאֲוִיר הָעוֹלָם, אָסוּר; וּלְאַחַר שֶׁיָּצָא לַאֲוִיר הָעוֹלָם, מֻתָּר מִיָּד.

A chick, so long as it has not gone out into the air of the world, is forbidden; and after it has gone out into the air of the world, it is permitted at once.

Se’if 2ב׳

בְּהֵמָה שֶיָּלְדָה, אִם יָדוּעַ שֶׁכָּלוּ לוֹ חֳדָשָיו, דְּהַיְנוּ ט' חֳדָשִׁים לְגַסָה, וְה' לְדַקָּה, מֻתָּר מִיָּד בַּיּוֹם שֶׁנּוֹלָד, וְלֹא חַיְשִׁינָן שֶׁמָּא נִתְרַסְקוּ (פי' נִכְתְּשׁוּ וְנִכְתְּתוּ) אֵבָרָיו מֵחֶבְלֵי הַלֵּדָה; וְאִם אֵין יָדוּעַ שֶׁכָּלוּ לוֹ חֳדָשָיו, אָסוּר מִשּׁוּם סָפֵק נֵפֶל עַד תְּחִלַּת לֵיל שְׁמִינִי.

An animal that gave birth: if it is known that its months were completed — that is to say, nine months for large cattle and five for small cattle — it is permitted at once, on the day that it was born, and we do not take account of the possibility that its limbs were crushed (that is, pounded and beaten) by the pangs of birth. And if it is not known that its months were completed, it is forbidden on account of a doubtful nefel (one born before its time, and so not viable) until the beginning of the eighth night.

Se’if 3ג׳

אֵין סוֹמְכִין עַל הָעוֹבֵד כּוֹכָבִים בִּגְדָיִים קְטַנִּים הַנִּקָּחִים מִמֶּנּוּ וְאוֹמֵר שֶׁהֵם בְּנֵי ח' יָמִים (תְּשׁוּבַת רַשְׁבָּ"א סי' רמ"ג).

One does not rely upon the idolater (oved kokhavim — the standing term for a gentile) with regard to small kids that are bought from him and he says that they are eight days old. (Responsum of the Rashba, siman 243)