Twilight foraging enables European shags to survive the winter across their latitudinal range

Børge Moe ,Aevar Petersen ,Francis Daunt ,Oskar Bjørnstad ,Tone Kristin Reiertsen ,Gunnar Þór Hallgrímsson ,Sindri Gíslason ,Sunna Björk Ragnarsdóttir ,Arne Follestad ,Mark A Newell ,Maria I Bogdanova ,Manuel Ballesteros ,Svein‐Håkon Lorentsen ,Kjell Einar Erikstad ,Nina Dehnhard ,Vegard Sandøy Bråthen ,Robert T Barrett ,Richard A Phillips ,Jens Åström ,Sarah Wanless ,Tycho Anker‐Nilssen

doi:10.3354/meps13697

Abstract

Species breeding at high latitudes face a significant challenge of surviving the winter. Such conditions are particularly severe for diurnal marine endotherms such as seabirds. A critical question is therefore what behavioural strategies such species adopt to maximise survival probability. We tested 3 hypotheses: (1) they migrate to lower latitudes to exploit longer day length (‘sun-chasing’), (2) they forage at night (‘night-feeding’), or (3) they target high-quality food patches to minimise foraging time (‘feasting’). We studied the winter migration and foraging strategies of European shags Phalacrocorax aristotelis from 6 colonies across a latitudinal gradient from temperate regions to north of the Arctic Circle using geolocators deployed over 11 winters. We found evidence for ‘sun-chasing’, whereby average southerly movements were greatest from colonies at higher latitudes. However, a proportion of individuals from higher latitudes remained resident in winter and, in the absence of daylight, they foraged during twilight and only very occasionally during the night. At lower latitudes, there was little evidence that individuals migrated south, nocturnal feeding was absent, and twilight feeding was infrequent, suggesting that there was sufficient daylight in winter. There was no evidence that winter foraging time was lowest at higher latitudes, as predicted by the ‘feasting’ hypothesis. Our results suggest that shags adopt different behavioural strategies to survive the winter across their latitudinal range, dictated by the differing light constraints. Our study highlights the value of multi-colony studies in testing key hypotheses to explain population persistence in seabird species that occur over large latitudinal ranges.

Highlights

At high latitudes, animals typically schedule breeding activities in the spring and summer to coincide with favourable environmental conditions (Lack 1968, Stearns 1992)
A critical question in understanding a species’ persistence across its range is how individuals breeding at higher latitudes survive the winter, in particular diurnal species that winter in regions experiencing the polar night where light levels are very low for a long period of the year
We found partial support for this prediction such that latitudinal displacement in December differed among colonies (LMM: n = 487, ΔlogLik = 50.6, ΔAIC = 70.3, χ2 = 104.2, df = 5, p < 0.001; Fig. 2A) and the greatest displacement was seen for the 2 northernmost colonies (Røst and Hornøya)

Summary

Introduction

Animals typically schedule breeding activities in the spring and summer to coincide with favourable environmental conditions (Lack 1968, Stearns 1992). Long-lived species must achieve this annual deadline by surviving the winter when faced with a number of challenges, including increased daily energy requirements, higher frequency of extreme weather events and, for diurnally foraging species, shortened day length (Newton 1998). The most predictable variation in physical conditions is day length and associated light levels, which show systematic changes with latitude and season. A critical question in understanding a species’ persistence across its range is how individuals breeding at higher latitudes survive the winter, in particular diurnal species that winter in regions experiencing the polar night where light levels are very low for a long period of the year

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Conclusion