Reproduction efficiency of the Steller's Sea Eagle on Sakhalin Island and the Lower Amur (Russia)

Abstract

Since the mid-2000s, we have been monitoring the status of two Haliaeetus pelagicus populations, breeding on Sakhalin Island (438 nesting territories) and the lower reaches of the River Amur (350 nesting territories), Russian Far East. The data were collected between 2004 and 2019, during 12 field seasons in each study area. The main focus was on reproductive vital rates: territory occupancy, the proportion of laying pairs, breeding success and brood size. Their combination determines how many fledglings the territory eventually produces (productivity and territory performance). Additionally, we estimated offspring loss by various causes. Finally, we recorded all H. pelagicus occurrences to characterise the population structure, i.e. the proportion of immatures and breeder-to-floater ratio. Our results showed that all characteristics varied greatly over time and space, and also varied across regions. The overall reproduction efficiency was quite low in both study areas: one nesting territory on the Lower Amur produces 0.51 fledglings per year, and 0.35 fledglings per year on Sakhalin Island. The mean productivity on Sakhalin Island was also lower than on the Lower Amur: 0.51 and 0.62 fledglings per occupied territory annually, respectively. This difference between study areas is mostly due to predation by Ursus arctos, which takes 18% of nestlings on Sakhalin but not on the Lower Amur. Apart from direct loss, U. arctos predation causes indirect effects on the H. pelagicus population by affecting territory occupancy and the proportion of laying pairs in the subsequent year. We revealed two linear temporal trends, both for the Sakhalin population (decrease in the proportion of laying pairs and increase in nestling mortality). However, more research and data analysis are needed to explain the low breeding performance in both study areas and guide conservation efforts to stabilise or recover the H. pelagicus populations.

Keywords

• brood size
• brown bear
• brown bear predation
• haliaeetus pelagicus
• nesting success
• population monitoring
• productivity
• sensitive data
• territory occupancy
• ursus arctos