Missing piece of top predator‐based conservation: Demographic analysis of an owl population on a remote subtropical island

Abstract

Top predators are frequently the target of conservation programs. Owls are such predators. However, previous studies of owls are biased to species occurring in temperate regions, whereas most owl species occur in tropical or subtropical regions and are understudied. Furthermore, owls are often endemic to islands and of unknown conservation status. Demographic data for such species are especially scarce although they are essential for initiating and promoting their conservation. As a case study of demographic analysis of owls in a tropical or subtropical area and on islands, we applied an integrated population model to 7‐year monitoring data (2012–2018) of the Ryukyu Scops Owl population on Minami‐daito Island, Japan. We used survival history data from 903 individuals, reproduction and sex ratio data from 213 broods, and count data of 2,526 individuals in total. Long‐term averages of annual survival rates were 0.73 for adult females and 0.74 for adult males, although the sexual difference was not significant. Sex ratio estimates fluctuated annually and long‐term averages were slightly skewed to males: 0.51 among fledglings, 0.54 among yearlings, and 0.52 among adults. Long‐term averages of population size were estimated to be 273.4 females and 296.8 males. The long‐term average of population growth rate was 0.98, suggesting a slightly declining trend. It was fortunate to recognize the declining trend during the early phase. Considering the general lack of fundamental ecological data on owls of tropical or subtropical areas and on islands, it seems likely that many endangered owl populations await conservation efforts.

Mate choice for body size leads to size assortative mating in the Ryukyu Scops Owl Otus elegans

“…Males carry food to their mates until the middle of the nestling period, and thereafter the parents share feeding duties (Murakami et al, 2022 ; Takagi & Akatani, 2011 ). There is no significant sexual difference in annual survival rate (Sawada, Iwasaki, Inoue, et al, 2021 ). The average body mass of adult males and adult females are 88.4 and 92.2 g, respectively, showing slight reversed sexual size dimorphism (Sawada, Iwasaki, Matsuo, et al, 2021 ).…”

“…For the survival rate, we constructed a CJS model with survival rate modeled by five fixed effects (Year, Sex, Age, Culmen length, Wing length) and detection probability modeled by two fixed effects (Sex, Researcher; Sawada, Iwasaki, Inoue, et al, 2021 ). To consider sex‐dependency of the effect of body size, regression coefficients for body size (Culmen length, Wing length) were modeled to be sex‐dependent.…”

“…As already mentioned, in this population, females show roaming dispersal pattern and settle in territories which are held by males. In addition, there are more males than females (Sawada, Iwasaki, Inoue, et al, 2021 ). For males, rejecting females visiting their territories may not be a good choice.…”

“…Top-ranked models with ΔAIC <2 (difference from minimum AIC smaller than 2) were model-averaged by model.avg to obtain model-averaged regression coefficients and corresponding p-values. Results were interpreted from the top model and the averaged model.For the survival rate, we constructed a CJS model with survival rate modeled by five fixed effects (Year, Sex, Age, Culmen length, Wing length) and detection probability modeled by two fixed effects(Sex, Researcher;Sawada, Iwasaki, Inoue, et al, 2021). To consider sex-dependency of the effect of body size, regression coefficients for body size (Culmen length, Wing length) were modeled to be sex-dependent.…”

Population Ecology 2023 Editorial

“…Demographic data are essential for initiating and promoting their conservation. The last image illustrates an integrated population model, which was applied to monitoring data of the Ryukyu Scops Owl population on Minami-daito Island, Japan (Sawada et al, 2021). The authors revealed the long-term averages of annual survival rates, annually fluctuating sex ratio, population size, and population growth rate.…”