[3][4][5] The nominate subspecies of West Peruvian screech owl is 20 to 22 cm (7.9 to 8.7 in) long and weighs 144 to 162 g (5.1 to 5.7 oz).

The former has a grayish facial disc with a black border, white brows above pale to golden yellow eyes, and small "ear" tufts.

[4] M. r. pacificus is smaller and lighter than the nominate, 18 to 19 cm (7.1 to 7.5 in) long and weighing 70 to 90 g (2.5 to 3.2 oz).

M. r. pacificus inhabits dry coastal scrub and deciduous forest, generally from sea level to 500 m (1,600 ft).

The nominate West Peruvian screech owl's song is "a trill of equally spaced notes increasing in volume toward the end".

That of M. r. pacificus is "a similar trill that rises in volume and drops slightly in pitch at toward the end".