Driver (PEQ  p. 131) sees a clear distinction between derōr “swallow” and sūs “swift,” but the differentiation is not quite so simple. These birds belong to widely separated orders, yet they have developed on similar lines; they take all their food in the air, where they spend most of their daylight hours, and are almost helpless on the ground; many of them nest in man-made situations. Palestine has some six members of the swallow family (Hirundinidae); European and red-rumped swallows; house, sand and two crag martins. Two are resident and the others migratory. Ancient writers are unlikely to have distinguished swifts and swallows precisely. Hebrew derōr also means “liberty.” It has been suggested that this is a more general word including all insectivorous birds that feed on the wing. Either swallow or swift would fit the contexts of Psalm 84:3 and Proverbs 26:2 equally well.
International Standard Bible Encyclopedia (1915)
A small long-winged bird of exhaustless flight, belonging to the family Hirundinidae. Deror means the bird of freedom, and as the swallow is of tireless wing, it has been settled upon as fitting the requirements of the text. In the passages where `aghur is translated "swallow," there is a mistake, that word referring to the crane. There is also a word, cuc or cic, that means a rushing sound, that is incorrectly translated "swallow," when it should be "swift" (Cypselus apus).
These birds are near relatives and so alike on the wing as to be indistinguishable to any save a close observer. Yet the Hebrews knew and made a difference. The swallow is a trifle larger and different in color. It remains all the year, while in numerous instances the swift migrates and is a regular sign of returning spring. The swallow is of long and tireless flight. The swift is so much faster that the sound of its wings can be heard when passing. The swallow plasters a mud nest under eaves, on towers, belfries, and close to human habitations. The swifts are less intimate, building in deserted places, under bridges and on rocky crevices. The swallows utter constantly a rather sweet low note; the swifts chatter harshly and incessantly at their nests. These differences are observable to the most careless people. Scientists separate the birds on account of anatomical structure also. Despite this, the birds are confused in most of our translations.
"Like a swallow or a crane, so did I chatter;
I did moan as a dove; mine eyes fail with looking upward:
O Lord, I am oppressed, be thou my surety"
Here `aghur is translated "swallow" and cuc "crane," which is clearly interchanging words, as the Arabic for "swift" is cuc, the same as the Hebrew. The line should read, "swift and crane." And another reason for changing swallow to swift, in this passage, lies in the fact that of the two birds the swift is the incessant and raucous chatterer, and this was the idea in the mind of Hezekiah when he sang his Trouble Song. Another incorrect reference is found in Jer 8:7: "Yea, the stork in the heavens knoweth her appointed times; and the turtle-dove and the swallow and the crane observe the time of their coming; but my people know not the law of Yahweh." Few swallows migrate. Returning swifts are one of the first signs of spring.
"As the sparrow in her wandering, as the swallow in her flying,
So the curse that is causeless alighteth not" (Pr 26:2).
This reference might apply to either, remembering always that the swift took its name from its exceptional flight, it being able to cover over 80 miles an hour. However, the swallow is credited with 800 miles in a night.
"Yea, the sparrow hath found her a house,
And the swallow a nest for herself, where she may lay her young,
Even thine altars, O Yahweh of hosts,
My King, and my God" (Ps 84:3).
Here is one instance, at least, where the swallow is at home and the translation correct. The swift might possibly have built in the temple: the swallow was sure to be there.