Scanning electron microscopy of eggshells of Cuculus lepidus of Java and Cuculus p. poliocephalus of the Asian continent revealed differences in ultra-structure incompatible with conspecific grouping. Eggshell ultra-structure, egg-type and colouration, and host selection of C. lepidus are to a high degree similar to C. saturatus, as are morphological characteristics and vocalizations (discussed elsewhere). It is therefore concluded that C. lepidus is a small tropical form of C. saturatus.

The hosts of C. lepidus are Phylloscopus trivirgatus and Seicercus grammiceps in Java, and S. castaniceps in Malaya. In Java there are two breeding seasons of C. lepidus, April-June and September—October, whereas in Malaya breeding is centred in February-May. During the second breeding season of C. lepidus on Java conspecific migratory races of C. saturatus occur sympatrically. Of the two migratory races of C. saturatus occurring on Java, the C. s. saturatus appears to be twice as common as the northern race horsfieldi. Collection dates indicate that the former race is more abundant in January and February and C. s. horsfieldi in September—November. Between the three races on Java there is no evidence of genetic exchange; residents are largely isolated by habitat, and wintering migrants are non-vocal.