a swallow: the small bird known for its quick, darting flight and for nesting in sheltered corners of buildings
This is the name of one particular bird, not a word for birds in general. Both times it appears it stands next to the ordinary word for a small bird (צִפּוֹר), and the pairing is the point: first the common bird anyone would notice, then the quick one that never seems to stop moving.
Each of the two verses leans on something the bird actually does. In Psalm 84:3 it is the nesting. The bird has found a place to settle her young right beside God's altars, and the worshipper who is shut out of the temple envies her that. In Proverbs 26:2 it is the flight. A curse with no cause behind it never lands, the way this bird keeps darting past without settling.
The name itself seems to come from that restless freedom of movement. Hebrew has an identically spelled word (דְּרוֹר) for a proclaimed release that frees people from debt and slavery and returns forfeited land to its owners, and most scholars trace both words back to one old idea of moving or flowing freely. On that reading this is the free flier, a bird named for how it moves rather than for how it looks or sounds.
Which bird exactly is not fully settled. The two candidates are the swallow and the very similar swift. Where a language keeps those apart, translators normally choose "swallow" here and give "swift" to a different Hebrew bird name, one spelled just like the word for a horse (סוּס).