All features: the most southernmost forms in Western Sahara and Mauritania.
Calls: those in the Azores and Madeira are of priority because so little is known of them. Sonograms of every form would be invaluable. Further studies in the Canaries could show whether the split on calls also has a north-south dimension.
Further notes on the forms studied here, particularly from other times of the year or at adjacent localities. For example, moult has reduced the sample sizes for features of the middle primaries at Tanger and Santander and observations from the Casablanca area of Morocco would add breadth to the studies on the Moroccan Atlantic coast.
Re-assessment of material, previously published or otherwise, in the context of the exact localities at which observations were made.