Measuring in length and weighing around , the western jackdaw is the second smallest member of the genus "Corvus". Most of the plumage is a shiny black, with a purple (in subspecies "monedula" and "spermologus") or blue (in subspecies "cirtensis" and "soemmerringii") sheen on the crown, forehead, and secondaries, and a green-blue sheen on the throat, primaries, and tail. The cheeks, nape and neck are light grey to greyish-silver, and the underparts are slate-grey. The legs are black, as is the short stout bill, the length of which is about 75% of the length of the rest of the head. There are rictal bristles covering around 40% of the maxilla and 25% of the lower mandible. The irises of adults are greyish or silvery white while those of juveniles are light blue, becoming brownish before whitening at around one year of age. The sexes look alike, though the head and neck plumage of male birds fades more with age and wear, particularly just before moulting. Western jackdaws undergo a complete moult from June to September in the western parts of their range, and a month later in the east. The purplish sheen of the cap is most prominent just after moulting.