Reduced abundance of brood parasites resulting from population impacts on host species by browsing Sika Deer

Abstract

Sika Deer Cervus nippon has been increasing in many forested areas of Japan during the prior 20 years, and it is known that the abundance of birds that use the forest understory has been decreasing in those areas. Since some of those birds are hosts to brood-parasitic cuckoo species, we analyzed monitoring data to look for a related decrease of those brood-parasitic species. The data used for the analysis were the Japanese Ministry of the Environment's “Monitoring Site 1000,” which are bird survey data collected at 20 sites from 2010 to 2019. In the areas with no or light deer browsing, there was no significant decrease in the abundances of Bush Warbler Cettia diphone, Sakhalin Leaf-warbler Phylloscopus borealoides and Eastern Crowned-warbler P. coronatus, all of which use the forest understory. The abundances of Lesser Cuckoo Cuculus poliocephalus and Himalayan Cuckoo C. optatus, brood parasites of the birds that inhabit the forest understory, did not show a particular decrease. There was a significant decline, however, in the abundances of these species in areas with heavy browsing by deer. For brood parasites, the host is an indispensable breeding resource, and the effect of the deer browsing impacts the cuckoos through the decrease of the host population.