I watch, and am as a sparrow alone upon the house-top - That is, I am "sleepless;" trouble drives sleep from my eyes, and I am kept awake at night - a common effect of grief. The following remarks, copied from the "Land and the Book" (i. 54, 55), will furnish all the illustration needful of this verse. "They are a tame, troublesome, and impertinent generation, and nestle just where you don't want them. They stop up your stove and waterpipes with their rubbish, build in the windows and under the beams of the roof, and would stuff your hat full of stubble in half a day if they found it hanging in a place to suit them . When one of them has lost its mate - a matter of everyday occurrence - he will sit on the house-top alone, and lament by the hour his sad bereavement."

As a sparrow alone - צפור tsippor, seems to be often used for any small bird, such as the swallow, sparrow, or the like. Bochart supposes the screech owl is intended.

I watch,.... Night after night, and take no sleep; cannot get any by reason of thoughtfulness, care, and trouble:
and am as a sparrow alone upon the housetop; or, "as a bird" (o); for there is no necessity of limiting it to a sparrow, to which the account does not seem so well to agree; for sparrows will not only perch on housetops and solitary places, but will make their nests in dwelling houses, and in places of public resort, as temples; hence David speaks of the sparrow finding an house near the altars of God, Psalm 84:3 and Herodotus (p) makes mention of sparrows and other birds making their nests in the temple at Branchides; which may serve to illustrate the text last mentioned: wherefore this may be understood of any solitary bird, and especially of the owl (q); the Jews had flat roofs upon their houses, and here birds of solitude would come and sit alone in the night season, to which the psalmist likens himself; being either forsaken by his friends and acquaintance; or, being in melancholy circumstances, he chose to be alone, mourning over his sorrowful state and condition.
(o) "sicut avis", Gejerus, Schmidt. (p) Clio, sive, l. 1. c. 159. (q) "--------tectoque prophanus Incubuit bubo" Ovid. Metamorph. l. 6. Fab. 8. "E tectis strix", &c. Tibullus, l. 1. Eleg. 5. v. 52.