Martin J.L. (1991) The Parus caeruleus complex revisited. ARDEA 79 (3): 429-438

A set of mensural and plumage pattern characters is analyzed by Standardized Principal Component Analysis over the entire range of the Parus caeruleus - Parus cyanus species complex. Clinal size variation in P. caeruleus extends into P. cyanus without discontinuity. The latter species has the largest specimens within the caeruleus - cyanus complex. Within both species one observes a trend towards smaller size, finer bills and longer tarsus along a north-east-south-west cline in P. caeruleus and a north-south cline in P. cyanus. Results are consistent with the hypothesis of a major role of changes in environmental conditions on mensural variation. Sample similarities based on plumage pattern show higher consistency with the probable phylogeny (see Vaurie 1957). Considering that the four major subspecies groups recognized are geographically and/or genetically isolated, the P. caeruleus - P. cyanus species complex could be described as a super-species comparised of four sister species consisting of the teneriffae (Lesson 1831), caeruleus (Linnaeus 1758), cyanus (Pallas 1770) and flavipectus (Severtrov 1873) subspecies groups recognized by Vaurie (1957).