Disturbance increases high tide travel distance of a roosting shorebird but only marginally affects daily energy expenditure

Hans Linssen,Karen L Krijgsveld,Bruno J Ens,Henk-Jan Van Der Kolk,Magali Frauendorf,Martijn Van De Pol,Andrew M Allen,Mitzi Jans

doi:10.1186/s40657-019-0171-8

Abstract

BackgroundAnthropogenic disturbance can negatively affect an animal’s energy budget by evoking movement responses. Existing research focuses mainly on immediate displacement as a disturbance effect, since this can be easily observed in the field. However, effects on movement over longer timescales are poorly examined and it is largely unknown if and to what extent they reflect immediate responses. Longer-term responses could for example be larger than immediate responses if birds, after disturbance, return to the original location and thereby travel twice the immediate disturbed distance.MethodsWe combined GPS tracking data with observational data to quantify the effects of anthropogenic (air force and walkers) and non-anthropogenic disturbances on distances travelled by roosting Eurasian Oystercatchers (Haematopus ostralegus) during the non-breeding season. We compared immediate displacement after a disturbance with distance travelled during the entire high tide period (longer-term response), while accounting for environmental factors. Additionally, we calculated energy expenditure due to disturbance based on observed disturbance frequencies.ResultsDisturbance resulted in an immediate displacement response of ~ 200 m (median). Air force disturbances tended to yield larger immediate responses than walker and, especially, than non-anthropogenic disturbances. Longer-term responses and immediate responses were approximately similar, suggesting that, over longer timescales, spatial disturbance effects in the study area remain confined to immediate effects. However, disturbances were infrequent (0.17 disturbances per bird per hour) and most disturbances were of natural origin (62%). Consequently, anthropogenic disturbance of roosting oystercatchers in the study area on average costs 0.08% of the daily energy expenditure.ConclusionsOur results suggest that immediate spatial responses to disturbance can be a useful proxy for spatial responses over longer timescales. Over the non-exhaustive range of conditions investigated, energetic consequences of spatial disturbance responses for an oystercatcher in the study area are marginal due to low disturbance levels.

Highlights

Anthropogenic disturbance can negatively affect an animal’s energy budget by evoking movement responses
For birds that follow a trajectory during high tide dictated by waterline fluctuation, disturbance does not necessarily incur extra flight distance compared to their normal trajectory if the disturbance causes them to move along that trajectory
Implications of disturbance We argue that the costs of anthropogenic disturbance for roosting oystercatchers are likely to be small in our study area

Summary

Introduction

Anthropogenic disturbance can negatively affect an animal’s energy budget by evoking movement responses. Longer-term responses could for example be larger than immediate responses if birds, after disturbance, return to the original location and thereby travel twice the immediate disturbed distance. Immediate spatial responses could be aggravated over the longer term when birds deviate from their normal spatial pattern and subsequently return to the initial location of disturbance (e.g. because their roosting or foraging area is located there). In those cases where longerterm shifts in spatial distribution of birds do occur Our current understanding of the impact of disturbance on shorebird populations is hampered by a lack of studies that link immediate individual and longer-term population spatial responses to disturbance

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Conclusion