Light pollution is increasing around the world and altering natural nightscapes with potential ecological and evolutionary consequences. A severe ecological perturbation caused by artificial lights is mass mortalities of organisms, including seabird fledglings that are attracted to lights at night on their first flights to the sea. Here, we report on the number of fledging short-tailed shearwaters Ardenna tenuirostris found grounded in evening and morning rescue patrols conducted at Phillip Island, Australia, during a 15-year period (1999–2013). We assessed factors affecting numbers of grounded birds and mortality including date, moon phase, wind direction and speed, number of visitors and holiday periods. We also tested experimentally if birds were attracted to lights by turning the lights off on a section of the road. Of 8871 fledglings found, 39% were dead or dying. This mortality rate was 4–8 times higher than reported elsewhere for other shearwater species, probably because searching for fledglings was part of our systematic rescue effort rather than the opportunistic rescue used elsewhere. Thus, it suggests that light-induced mortality of seabirds is usually underestimated. We rescued more birds (dead and alive) in peak fledging, moonless and windy nights. Mortality increased through the fledging period, in the mornings and with increased traffic on holiday periods. Turning the road lights off decreased the number of grounded birds (dead and alive). While moon, wind and time are uncontrolled natural constraints, we demonstrated that reduction of light pollution and better traffic management can mitigate artificial light-induced mortality.

Competing interests: One of the authors (André Chiaradia) currently serves as an Academic Editor or Guest Editor for this journal. Please note that this does not alter the authors' adherence to PLOS ONE Editorial policies and criteria.

Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. The authors confirm that all data underlying the findings are fully available without restriction. Relevant data are within the paper and Supporting Information files, and rescue campaigns data are available at: http://digital.csic.es/handle/10261/102107 .

Introduction

Natural parks aim to preserve natural conditions and encourage sustainable use by the public. Public visitation and engagement are essential for generating support and economic resources for nature conservation, but any detrimental effects that visitors have on biodiversity need to be managed (e.g. direct disturbance, road traffic, or habitat fragmentation; [1]). Light pollution is a frequently overlooked consequence of human visitation, mainly because a majority of visits to park and reserves occur during daylight hours. This pollution source can induce evolutionary and ecological disruptions on a wide range of biological processes, and thus, add to a plethora of threats challenging biodiversity conservation [2]–[4]. From a conservation perspective, one of the most severe perturbations caused by light pollution is episodes of mass mortality of organisms from different taxa, including birds, sea turtles and numerous insect species (e.g. [5]–[8]).

The conservation status of seabirds is deteriorating faster than any other group of birds [9] with petrels (Order Procellariiformes) occupying the top in the ranking [10]. Burrow-nesting petrel species (including shearwaters and storm-petrels) are nocturnally active at their breeding colonies, and their fledglings leave their nests at night, when they can be negatively affected by artificial lighting [11]. Worldwide, thousands of birds are attracted to lights every year during their first flights from their nests to the open ocean [12]–[17]; a phenomenon called ‘fallout’ [18]. Some fledglings may actually reach the ocean successfully but are attracted by the coastal lighting back onto the land [19]–[21]. Fledglings are vulnerable to injury or death by collisions with human infrastructure and once grounded, to predation or becoming road casualties. Through rescue campaigns conducted by the public, NGOs and local and regional governments, many fledglings are released back into the wild. However the basis for the attraction to or disorientation by fledglings requires more research [10]. Information coming from rescue campaigns can be crucial in shedding light on the extent of this ecological problem and its impact on these cryptic species (e.g. [22]–[25]).

We used information gathered during short-tailed shearwater Ardenna tenuirostris rescue campaigns conducted at Phillip Island, Victoria, Australia, from 1999 to 2013 to report the number of affected birds and the temporal and spatial patterns of groundings. Phillip Island differs from other locations where fallout has been studied (e.g. Azores, Canary Islands, Hawaii or La Reunion Island), in the larger size of the shearwater breeding population and the lower altitude and lower light pollution levels. Phillip Island is an “Important Bird Area” supporting more than 1% of the global populations of both short-tailed shearwaters and little penguins Eudyptula minor [26]. Ecotourism is a major economic activity on the island. Visitors can watch little penguins crossing the beach at evening at the Penguin Parade (Phillip Island Nature Parks, PINP, www.penguins.org). The activity attracts over half a million visitors each year, providing crucial financial support to the conservation and management of island wildlife through ecotourism [27]. However, it also increases road traffic at certain hours (approx. 2 hours before and after sunset) and consequently also light pollution coming from road and vehicle lights [28]. Visitors, mainly coming by road from Melbourne, have to cross a bridge to get onto the island and drive to the Penguin Parade (Figure 1).

PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Figure 1. Phillip Island map showing the road where rescue patrols are conducted, light pollution levels and the main Short-tailed shearwater (Ardenna tenuirostris) breeding colonies. Light pollution levels are taken from a cloud-free composite of VIIRS night time lights corresponding to April and October of 2012 and produced by the Earth Observation Group, NOAA National Geophysical Data Center (available at http://ngdc.noaa.gov/eog/viirs/download_monthly.html). The maximum light pollution levels for Phillip Island and Wonthaggi desalination plant are in brackets (nW/sr*cm2). https://doi.org/10.1371/journal.pone.0110114.g001

We have assessed potential factors determining the number and the light-induced mortality of grounded birds by artificial lights per night. Short-tailed shearwater fledglings seem to need strong winds to take off on their initial flights [29], although no detailed studies have been conducted on this issue in petrels [30]. Therefore we anticipated wind speed could have a positive relationship with the number of birds grounded and the proportion of dead birds (mortality). Given the location of breeding colonies in relation to the roads where rescue patrolling is conducted (Figure 1), we expected that fledglings would drift towards artificial lights with southerly or westerly winds. We also tested the potential effect of the holiday periods or visitor numbers, expecting higher number of birds grounded during nights with higher numbers of vehicles. We included moon phases and date in the analyses as a majority of shearwaters are grounded during moonless nights and around the peak fledging period elsewhere [12], [14]–[16]. Finally, the relative calm water of the adjoining bay, Western Port, may be an important area for fledglings as they can rest and preen before starting migration to North Pacific Ocean. The bridge to the island may constitute a barrier for fledglings coming into or out of the bay. Aiming to mitigate mortality, PINP in collaboration with VicRoads (State Government department with responsibility for roads in Victoria) and SP AusNet (VicRoads lighting contractor) turned lights off on the bridge to the island during some days of the fledging period. We evaluate whether this lighting management had any effect on the attraction of fledglings to lights.