Breeding ecology of Eastern Crowned Leaf Warbler and brood parasitism by the Oriental Cuckoo

Abstract

We studied the breeding ecology of Eastern Crowned Leaf Warbler Phylloscopus coronatus and brood parasitism by the Oriental Cuckoo Cuculus optatus in western Hokkaido for 17 years. Of 135 nests found, most were on the ground (65.2%), or on the collapsed stems and leaves of bamboo grass (32.6%). The average clutch size of 6.4, was larger than that found in previous studies, perhaps because of the latitude of the study site. The average incubation period was 11.7 days and the average nestling period was 10.9 days. Both male and female warblers contributed to feeding their nestlings, but the parental division of labor varied greatly among nests and over time; the average feeding ratio of females was higher in the later (45.5%) nestling period than in the earlier (34.8%) nestling period. The probability of survival from the start of incubation to fledging, as calculated using the Mayfield method, was 0.411 (0.781 for the incubation period and 0.526 for the nestling period). Predation of warblers was significantly higher (33.6%) during the nestling period than during the incubation period (13.0%). The average rate of brood parasitism by the Oriental Cuckoo was 16.3% throughout the study, but varied from a high of 37.5% to a low of 0.0% over the 11 years from 2012 onwards. All Oriental Cuckoo eggs were plain reddish-brown, and though strongly contrasting with the white eggs of East Crowned Leaf Warblers, the warblers did not reject either cuckoo eggs or nestlings . The average incubation period of the cuckoo eggs was 11.5 days, the nestling period was 16.3 days, and the fledging success rate was 36.4%.