pasero

It is not pasi plus -er-, and it matters. Every piece of that reading is available: pasi at 310 is to pass, and -er- is the suffix for one fragment of a mass, giving sablero a grain of sand and monero a coin out of money. So paser-o looks like it should mean a small piece of passing, which is nonsense, and the seam is entirely accidental. A sparrow is one Latin root, passer-, and the double s of the Latin flattens to a single one in the Esperanto spelling.

Fit it into the bird set you already have. Aglo the eagle, korvo the crow, cigno the swan and now the smallest of them. That is the pattern of this half of the frequency list: the big and symbolic birds come first because the old texts are full of them, and the ordinary garden ones arrive late, which tells you something about the corpus rather than about birds.

The idiom carries over. A pasero in Esperanto does the same work it does in English and in most of Europe: it is the everyday, unglamorous, plural bird, the one in proverbs about small things. Pli valoras pasero en mano ol aglo en aero is the Esperanto version of the bird in the hand, and it is worth knowing because it turns up in beginners' readers constantly.