Activity patterns at the Arctic Circle: nocturnal eagle owls and interspecific interactions during continuous midsummer daylight

Ane Eriksen,Petter Wabakken

doi:10.1111/jav.01781

Abstract

Circadian rhythms result from adaptations to biotic and abiotic environmental conditions that cycle through the day, such as light, temperature, or temporal overlap between interacting species. At high latitudes, close to or beyond the polar circles, uninterrupted midsummer daylight may pose a challenge to the circadian rhythms of otherwise nocturnal species, such as eagle owls Bubo bubo. By non‐invasive field methods, we studied eagle owl activity in light of their interactions with their main prey the water vole Arvicola amphibius, and their competitor the white‐tailed eagle Haliaeetus albicilla during continuous midsummer daylight on open, treeless islands in coastal northern Norway. We evaluated circadian rhythms, temporal overlap, exposure, and spatial distribution. The owls maintained a nocturnal activity pattern, possibly because slightly dimmer light around midnight offered favourable hunting conditions. The eagles were active throughout the 24‐h period as opposed to the strictly diurnal rhythm reported elsewhere, thus increasing temporal overlap and the potential for interference competition between the two avian predators. This may indicate an asymmetry, with the owls facing the highest cost of interference competition. The presence of eagles combined with constant daylight in this open landscape may make the owls vulnerable to interspecific aggression, and contrary to the available literature, eagle owls rarely exposed themselves visually during territorial calls, possibly to avoid detection by eagles. We found indications of spatial segregation between owls and eagles reflecting differences in main prey, possibly in combination with habitat‐mediated avoidance. Eagle owl vocal activity peaked in the evening before a nocturnal peak in visual observations, when owls were active hunting, consistent with the hypothesis of a dusk chorus in nocturnal bird species. The owls may have had to trade‐off between calling and foraging during the few hours around midnight when slightly dimmer light reduced the detection risk while also providing better hunting conditions

Highlights

Animal activities often follow cyclic patterns, which are adaptations to predictable changes in the biotic and abiotic environment (Kronfeld-Schor et al 2017)
We evaluated the temporal overlap between owl and vole activity and between owl and eagle activity using the methods developed by Ridout and Linkie (2009) using the R package ‘overlap’ (Meredith and Ridout 2014, 2017)
We found that eagle owls were clearly nocturnal during 24-h midsummer daylight, with the highest activity occurring when the sun was at its lowest around midnight, and with minimal activity in the middle of the day (Fig. 1a)

Summary

Introduction

Animal activities often follow cyclic patterns, which are adaptations to predictable changes in the biotic and abiotic environment (Kronfeld-Schor et al 2017). Animals may e.g. increase their access to food by increasing temporal overlap with prey species (Jenny and Zuberbuhler 2005, Penido et al 2017) and decreasing overlap with competitors (Halle 2000, Gerber et al 2012), and they may reduce predation risk by minimizing overlap with predators (Nelson and Vance 1979) To adjust their activity patterns to cyclic environmental conditions, animals have evolved internal biological clocks that are set to environmental synchronizers (or Zeitgebers) the major of which is the light-dark cycle (Daan and Aschoff 1975, Halle 2000, Nouvellet et al 2012, Kronfeld-Schor et al 2017). The activity patterns of such species during the Arctic summer may depend on the plasticity of the internal timing system, both of the species in question and of species with which it interacts (Speakman et al 2000, Kronfeld-Schor et al 2017)

Methods

Results

Conclusion