Albert de Jong
From the Rarities Committee's files - Identification of Caspian Gull. Part 2: phenotypic variability and the field characteristics of hybrids, by Chris Gibbins, Grzegorz Neubauer and Brian Small, IN: BB 104/2011.
Fig. 9. PCA biplot showing sample first-winter Herring Larus argentatus and Caspian Gulls L. cachinnans and a number of proven and suspected hybrids. Each circle represents a sample bird. Together, axes l and 2 capture 73.4% of the variance in the sample data. The polygon to the right groups sample Caspian Gulls, while that to the left groups Herring Gulls. Hybrid birds are highlighted in red; numbers relate to plates that show examples of some of these hybrids. Codes for traits are: Bill=bill length:depth ratio, Pproj=length of primary projection, TM=extent of tertial moult, GCovM=extent of greater-covert moult, MCovM=extent of median-covert moult, ScapM=extent of scapular moult, GCovPat=greater-covert pattern, Tpat=tertial pattern, Legs=leg length,Vbul=presence of ventral bulge, Darkness=extent of streaking on head and body, and ScapP is the pattern on second-generation scapulars.