Dark heterochromia in adult masked lapwings is universal, asymmetrical and possibly slightly sexually dimorphic

Abstract

Variations in eye colour in birds are poorly documented. We measured and characterised eyes of 25 birds in the hand (16° × 22.5° segments per eye), using standardised and scaled images, and examined observations of 1 marked individual through time. We describe universal and extensive dark heterochromia (non-uniform colouration of the eye) in adult masked lapwings Vanellus miles novaehollandiae in our study population. Asymmetrical within-eye heterochromia has not previously been recorded for Charadridae, but in a review of images of eyes in the genus Vanellus, we found 45% of the 22 species had detectable heterochromia. There was a tendency (p = 0.051 after false discovery rate correction) for males to have slightly less black in the upper posterior part of the eye, a difference not distinguishable enough to aid sexing in the hand. We suggest standardised images of captured birds would underpin a more nuanced understanding of the occurrence of subtle heterochromia among birds, which may permit phylogenetic analyses directed at exploring possible evolution or function of such eye pigmentation.