Visually-oriented predators, such as seabirds, are highly light dependent, and thus their presence and activity under continuously dark conditions of Arctic polar night pose a number of questions about the strategies and mechanisms they use to find prey. Here, opportunistic observations of the behaviors of Thick-billed Murres (Uria lomvia; n = 4) and juvenile Black Guillemots (Cepphus grylle; n = 5) were made in the ocean around Spitsbergen Island, Svalbard Archipelago, off the coast of Norway. These observations were made between 15–23 January 2014–2017 during the darkest period of the polar night. Underwater observations recorded on 23 January 2014 and 19–20 January 2015 revealed that individual birds seemed to be attracted to artificial light. They actively foraged in the sea within the beam of scuba diver lights and harbor lamps indicating that artificial light may create additional feeding opportunities for seabirds present in the area. Other observations of Dovekies (Alle alle; n = 2) made on 15–16 January 2016 may indicate that not all seabird species exhibit such an adaptable behavior. Various seabird reactions might be caused also by different age and intra-specific variation among individuals; however, due to the limited number of observations, future studies are needed to increase our understanding of these behaviors.
The ambient light regime (irradiance, spectral composition and day length) plays an important role in the behavior of seabirds, affecting activities such as feeding, migration or reproduction (Wiese et al. 2001; Montevecchi 2006). For visual predators, the efficiency of foraging is strongly influenced by the level of available light, which is important for the detection of prey (Jetz et al. 2003). In other studies involving seabirds, light-limited foraging patterns were recorded for five penguin species (Wilson et al. 1993), Imperial Shags (Phalacrocorax atriceps) and European Shags (P. aristotelis) (Wanless et al. 1999).
In contrast to these observations, Gremillet et al. (2005) found that Great Cormorants (Phalacrocorax carbo) are able to forage in the dark and do not modify their foraging rhythms according to light levels. Some seabird species have learned to use nocturnal light (moonlight and starlight) and frequently perform foraging dives at night (Regular et al. 2011). For example, Swallow-tailed Gulls (Creagrus furcatus) maximize their foraging activity during darker periods of the lunar cycle (Cruz et al. 2013) in response to the availability of their prey, which migrates toward deeper water during the day and to the surface at night (a phenomenon called diel vertical migrations; Zaret and Suffern 1976). Red-legged Kittiwakes (Rissa brevirostris) also concentrate their foraging during nighttime since their main prey, lanternfish (Myctophidae), are available at the surface during this period (Kokobun et al. 2015).
Even night foraging seems to be at least partially visually guided (Regular et al. 2011). Nocturnal birds have eye shapes that are maximized for visual sensitivity (Hall and Ross 2007) and are considered to be highly light sensitive (Montevecchi 2006). In addition to using moonlight, starlight (Johansen et al. 2001; Regular et al. 2011) or bioluminescence (Imber 1975) to detect their prey, seabirds are also known to exploit foraging grounds exposed to artificial light (Montevecchi 2006).
The continuous darkness lasting over 3 months during the Arctic polar night offers a perfect natural situation for observations and studies on light attraction in seabirds. Perception of polar ecosystems during the polar night has changed. Many of the ecosystem components remain active and ready for the reappearance of the sun (Berge et al. 2015a). Among these are seabirds - an important link in energy transfer in polar ecosystems (Zmudczyńska-Skarbek et al. 2015). The presence and activity of seabirds during Arctic polar night was reported for the first time only recently (Table 1). The objective of this paper is to report on opportunistic observations of seabirds foraging under artificial light conditions during the Arctic polar night.
Spitsbergen (78° 45′ 0″ N, 16° 0′ 0″ E) is the largest Island of the Svalbard Archipelago, located off the coast of Norway (Fig. 1). The western fjords of the island are under the influence of two current systems: 1) the warm, salty West Spitsbergen Current that branches off the North Atlantic Current; and 2) the colder nutrient-poor waters originating in the Arctic Ocean carried by the East Spitsbergen Current (Loeng 1991). The salinity regime in the surface water varies from 28 Practical Salinity Units (PSU) to 34 PSU (Nilsen et al. 2008), while the sea temperature ranges from -1.4 °C to +7 °C (Włodarska-Kowalczuk et al. 1999). The surface inshore fjord waters of the West Spitsbergen Shelf typically freeze for a few months in winter (November–May), while the outer part of the fjord remains generally ice free (Svendsen et al. 2002; Nilsen et al. 2008). In 2014–2017, the sea did not freeze in the study area. There is apparent darkness starting from November through February on Spitsbergen at latitudes above 78° N, while in northern Norway (69° N) and West Greenland (69° N), where foraging behavior of wintering Great Cormorants was observed (Johansen et al. 2001; Gremillet et al. 2005; Table 1), there is still considerable ambient light during the daytime (Berge et al. 2015b).
Seabird observations were made while performing other tasks in the field and thus were opportunistic. All of the observations took place during the darkest period and the middle of the polar night (15–23 January) in four consecutive years (2014–2017; Table 2). In the first two years, underwater observations were done while shallow diving (0–7 m, consisting of approximately 10 40- to 45-min dives each year) in Kongsfjorden in the vicinity of Ny-Ålesund harbor (78° 55′ 43.95″ N, 11° 56′ 00.96″ E; Fig. 1). In 2014, seabird behavior was recorded on video camera (see Acknowledgments section for details), while in 2015 it was captured on still photographs (Fig. 2). On each occasion, the two-diver teams were equipped with strong video lights visible on the surface and underwater from a distance of several meters. Apart from the light originating from divers, there were some other artificial light sources from harbor buildings. In 2016, seabirds were observed from on-board R/V Helmer Hanssen in an uninhabited area of Smeeren-burgfjorden (79° 43′ 41.23″ N, 10° 58′ 16.68″ E; Fig. 1) with no artificial light sources except those of the ship. In 2017, observations were made in Isfjorden from the boat (M/S Farm) moored inside the Longyearbyen harbor (78° 14′ 23.00″ N, 15° 32′ 37.45″ E; Fig. 1). The sea water was lit by a hand flashlight and harbor lamps.
Both underwater and land observations indicated that Thick-billed Murres in 2014 (n = 1) and 2016 (n = 3) and Black Guillemots in 2015 (n = 2) and 2017 (n = 3) (Table 2) were foraging in the beam of artificial light (divers' lamps, flashlights, harbor lamps). During one of the 10 dives in 2014, an individual Thick-billed Murre was observed. It approached divers and dived around them near the sea surface and up to 3–5 m depth catching larger zooplankton organisms (krill, Thysanoessa spp.) that were easily visible in the illuminated water. On 19–20 January 2015, a juvenile Black Guillemot (n = 2) was observed during two of the 10 dives. The individual followed the divers and the beams of light for 2–3 min, diving several times to a depth of ~6–7 m searching for prey and catching small fish within the seaweed. Between 15–19 January 2017, juvenile Black Guillemots (n = 3) were observed from the boat moored in the harbor. These individuals appeared to be attracted by the boat lights/flashlights, approached and started to forage on plankton (Common Clione, Clione limacina) when the water was illuminated. During observations in Smeerenburg on 15–16 January 2016, Thick-billed Murres (n = 3) were attracted by light, but Dove kies (n = 2) appeared to actively keep their distance and were always out of the range of the light. All of the birds (Black Guillemots, Thick-billed Murres and Dovekies) were recorded in Spitsbergen under continuously dark environmental conditions (Berge et al. 2012, 2015a, 2015b) (Table 2).
Literature summary of seabird observations during Arctic polar night (December–January). n/a = not applicable.
Observations have indicated that artificial lights create additional feeding opportunities for both Thick-billed Murres and Black Guillemots (Montevecchi 2006). However, to date, the effect of light on Arctic seabirds has remained generally unknown (Humphries and Huettmann 2012), and no direct underwater observations of these species actively foraging during the darkest period of polar night had been conducted. Feeding in areas exposed to artificial nocturnal lighting is generally considered favorable for seabirds as this enhances food supply by attracting fish and zooplankton to the surface waters where it can be easily caught (Burke et al. 2005; Montevecchi 2006). It also provides sharp contrast against the completely dark environment that helps to target the prey.
Factors such as weather, season, ambient solar light conditions and lunar phase strongly influence attraction to sources of light. It is stronger, for example, during the night or at times of low cloud cover, and weaker on bright clear nights with a full moon (Telfer et al. 1987; Montevecchi 2006). Artificial lights would thus be expected to act as an especially intense stimulant for seabirds during polar night. The positive reaction of the seabirds to light could be explained by the need to detect bioluminescent prey items (Berge et al. 2012); seabirds feeding at night on vertically migrating bioluminescent prey are photosensitive, being attracted to light points in the sea (Imber 1975). Indeed, in studies on light-induced seabird strikes on vessels in Southwest Greenland (Merkel and Johansen 2011), a higher frequency of bird strikes was found during the darkest mid-winter period with reports of small numbers of Black Guillemot and Thickbilled Murre being killed in this way.
While attraction to artificial light has been observed in many different bird species (Harris et al. 1998; Rodriguez and Rodriguez 2009; Miles et al. 2010), not all seabirds respond positively to light. Dovekies, which were thought to be light attracted (Wiese et al. 2001), seemed to actively avoid artificial light during our two observations. However, due to the limited number of observations, additional studies are needed to confirm this behavior. It is likely that there is some contribution from behavioral variation among individuals influencing their interactions with the environment (Roche et al. 2016). Individuals among one species can differ in personality and degree of behavioral plasticity (Roche et al. 2016). Age can also influence the attraction of birds to light. Some fledgling auks (Alcidae) are possibly more attracted to artificial light than adults (Montevecchi 2006). Older individuals may have learned to avoid artificial light sources, associating them with threats, while immature birds may be confused by incorrect visual orientation (Telfer et al. 1987) or environmental inexperience (Montevecchi 2006). Further studies could resolve this with more observations of a greater number of individuals. Interestingly, in the area where both the Dovekies and Thick-billed Murres were seen, food was found in their stomachs even though no other artificial light sources were present (Berge et al. 2015a; this study). This leaves open the question as to how these birds obtain their food and what strategies are used to survive in the unfavorable conditions of the Arctic polar night.
Study observation summary of seabirds during Arctic polar night (January).
The video of Thick-billed Murre behavior recorded in 2014 is available at http://youtu.be/Kq8TqQ_Ckqs. This study was performed according to and within the regulations enforced by the Norwegian Animal Welfare authorities, and no specific permissions were required, except for hunting of seabirds in 2016, which was conducted with permission from the Governor of Svalbard, given in accordance with the environmental protection regulations for Svalbard. The authors are grateful to Jakub Szuster, who assisted in diving, and two anonymous reviewers whose comments led to an improved manuscript. The Research Council of Norway (Marine Night, 226417) and the Polish Ministry of Science and Higher Education (W157/Norway/2013) provided funding. Kaja Ostaszewska has been supported as a Ph.D. student by the Centre for Polar Studies, KNOW - Leading National Research Centre, Poland.