Food-niche Partition and Sexual Dimorphism of Northern Boobooks (Ninox japonica) and Oriental Scops Owls (Otus sunia) in Korea

Abstract

Two small forest owls, the Oriental Scops Owl (Otus sunia) and the Northern Boobook (Ninox japonica), are among the common breeding raptors in the Republic of Korea. However, very limited ecological information is available on these nocturnal raptors. In this study, I focused on two aspects of these owls ecology: sexual dimorphism and food-niche partitioning during their early breeding period. The morphometric study was aimed to understand sexual dimorphism in these two small owls through external measurements, and to develop sex-discriminant functions for both species. The study of food-nice partitioning of two sympatric owls used stable isotope analysis (SIA) to determine trophic levels and estimate prey type selection. From both morphometric information and individual SIA results, I questioned how body size can effect individual prey use, and further its relationship with the evolution of reversed sexual dimorphism in raptors. From April to May in 2014 and 2015, 30 Oriental Scops Owls and 10 Northern Boobooks were captured using mist nets after sunset between 19:30 to 00:30 at Gwang-reung Forest and Gwan-ak Arboretum in Gyeonggi-do province. Blood samples and measurements were taken from captured owls. Moreover, 16 scops owls and 22 boobooks admitted to or kept in four wildlife rescue centers were measured and sampled for the morphometric study. For diet reconstruction, potential prey items (ground insects, moths, mice, and birds) were collected and analyzed to estimate the diet composition of each owl species through multi-source mixing model. In terms of morphology, Oriental Scops Owls showed significant but subtle reversed sexual dimorphism whereas, Northern Boobooks had no clear evidence of sexual dimorphism. Bill depth, bill width, tail length and tarsus length were selected for the stepwise discriminant function in Oriental Scops Owls, and they sexed of 79% the birds correctly by re-substitution. The stepwise discriminant function for Northern Boobooks using head length and bill length to skull, correctly classified the sexes with about 70% accuracy. Given the low power of morphometric sex discrimination as well as non-obvious sexual size dimorphism, additional techniques such as examination of brood patch or even molecular methods, are required for reliable sex determination. SIA showed significant distinctions between the two species, both in carbon and nitrogen isotope ratios. Northern Boobooks had higher values in nitrogen and carbon isotope ratios, and greater variance in carbon isotope ratios than Oriental Scops Owls. According to the multi-source mixing models, Northern Boobooks mainly consumed vertebrate prey, especially birds, but Oriental Scops Owls utilized mainly ground insects followed by birds, mice, and moths. Each individual owls overall body size index was extracted from principal component analysis, and compared with individual trophic level (nitrogen isotope ratio). Only Oriental Scops Owls showed significant correlation with smaller individuals consuming higher trophic level diets. In conclusion, Northern Boobooks and Oriental Scops Owls demonstrated distinct food niche separation in a sympatric habitat. In particular, the larger bodied Northern Boobooks occupied a higher trophic level with more flexible foraging habits, and they selected diverse prey sources from forests to open environments, whereas the smaller Oriental Scops Owl belonged to a lower trophic level and was a more specialized forager relying on forest ecosystems. Furthermore, individual trophic level was found to be related to the body size only for the sexually dimorphic Oriental Scops Owls, but not in the monomorphic Northern Boobooks. These findings partially support the small male hypothesis, in Oriental Scops Owls that smaller males are efficient foragers which is beneficial in reproductive success and thus more adaptive in raptors with reversed sexual dimorphism