Reference for Kinnim 2:1

קֵן סְתוּמָה שֶׁפָּרַח מִמֶּנָּה גוֹזָל לָאֲוִיר, אוֹ שֶׁפָּרַח בֵּין הַמֵּתוֹת, אוֹ שֶׁמֵּת אַחַד מֵהֶן, יִקַּח זוּג לַשֵּׁנִי. פָּרַח לְבֵין הַקְּרֵבוֹת, פָּסוּל וּפוֹסֵל אֶחָד כְּנֶגְדּוֹ, שֶׁהַגּוֹזָל הַפּוֹרֵחַ, פָּסוּל וּפוֹסֵל אֶחָד כְּנֶגְדּוֹ:

An unspecified pair of birds, from which one flew off, or that flew among birds designated to die, or if one of the pair died, a second bird should be bought for the remaining bird. If it flew off among birds that were set to be brought as sacrifices, it becomes invalid and invalidates its matching bird [the other bird of its pair], since a bird that flies away from its pair is invalid and makes its matching bird invalid.

Jastrow

Jastrow