Reference for Kinnim 2:4

קֵן סְתוּמָה וְקֵן מְפֹרֶשֶׁת, פָּרַח מִן הַסְּתוּמָה לַמְפֹרֶשֶׁת, יִקַּח זוּג לַשֵּׁנִי. חָזַר, אוֹ שֶׁפָּרַח מִן הַמְפֹרֶשֶׁת רִאשׁוֹן, הֲרֵי כֻלָּן יָמוּתוּ:

An unspecified pair of birds [one of which is a burnt-offering and the other is a sin offering] and a pair of specified birds, if a bird from the unspecified birds flew into the pair of specified birds he [the owner] must bring a second bird for the second one [remaining unspecified bird]. Or if a bird flew from the original specified birds [to the unspecified birds], they must all be left to die.

Jastrow