TT #18 · Parashas Noach · Second Aliyah · Passuk 7:3
Of the birds of the sky also, seven pairs, male and female, to keep seed alive upon all the earth.
Torah Temimah points to the derasha excluding aged or sterile animals (“lechayot zera”) (Avodah Zarah 6a).
Sefaria — Avodah Zarah 6a · E-Daf — Avodah Zarah 6a
“To keep seed alive” — to exclude the aged and the sterile (who cannot produce offspring).
Custom (exclusion of non-reproductive animals from the purpose of preserving seed).
Only animals capable of reproducing were taken; aged and sterile animals were excluded so that seed could truly be kept alive.