Microspectrophotometric (msp) measurements were made of retinal oil droplets of 15 species of birds from 5 orders. The droplets were assigned to six categories on the basis of their cut-off wavelengths. Counts of oil droplets from the retinae of different species revealed large variations in the proportions of oil droplets of different categories. Cluster analysis was used to demonstrate relationships between 12 species of birds on the basis of their oil droplet complements. This analysis linked species in ways which were best explained by ecological factors and which only sometimes reflected phylogeny.