Differences found between the dark form and the other forms include:
In the Cantabrican form, a reddish-brown mantle colour; dull rumps and tail bases with extensive mottling; bills with pale bases; and later primary moult with most still growing P9 and P10 in early August of their second calendar year when other forms have mostly completed this stage. At least some show a broader white tip to the tertials with internal notches. Such differences are presumably the influence of argenteus except for the reddish-tinged mantle which is presumably the influence of michahellis.
In Iberia, a reddish-tinged mantle is frequent (80%) and many (50%) particularly in north Portugal show dull rumps and tail bases with extensive mottling. The reddish-tinged mantle is presumably the influence of michahellis and the dull tails the influence of argenteus.