Once, offshore islands were a safe place to breed for a number of bird species. However, times are changing. Researchers from BResearch, Groningen report a dramatical increase of summer predation by polar bears.

The small offshore islands along the western coastline of Svalbard are a remarkable habitat hosting an array of bird species that seek a safe place to breed. However, times are changing. During a long-term population study on barnacle geese between Isfjorden and Bellsund (initiated in the 1970s; run by University of Groningen, The Netherlands, Wildfowl and Wetlands Trust, Slimbridge, UK, and BResearch, Groningen), the researchers witnessed developments that might severely impair the attractiveness of these islands.

Where the sea is an effective barrier to keep arctic foxes from the islands, it would not discourage another potential predator of eggs and newly hatched chicks, the polar bear. However, polar bears used to be rare along the west coast of Svalbard in summer, and only occasionally it was suspected that low breeding success in the most eastern colony of the study area was due to bear predation.

Any direct summer observations on polar bears collected in the 1970s and 1980s were limited to the last days of May and very first days of June when bears were associated with pack ice along the coast without showing any interest in the islands where the researchers studied breeding performance of barnacle geese. After the 1980s no polar bear at all were observed, and it seemed that with diminishing sea ice, bears avoided the study area in summer.

A couple of years ago this situation changed drastically, when the summer bears returned. However, this time not in early summer, but from late June onwards (Figure 1). Moreover, bears came to visit the offshore study islands. For birds breeding on the islands, effects appear dramatic. For example, in 2009 the first bear came to the main study colony on 27 June (Figure 2). At that moment, 200 pairs of barnacle geese and a similar number of eiders were incubating their eggs. When the polar bear left the colony some 20 hours later, only 6 % of the nests remained. The contents of the other nests had been taken by the bear, or by gulls benefitting from the confusion caused by the bear.

Figure 2. Routes of three polar bears successively visiting the main study colony, 2009. Locations of barnacle goose nests are indicated. Only 5 nests escaped predation and produced at least one gosling. Positions of bears were plotted each 2–5 min. Bear 4 checked the island without finding anything. The island is 350 m long.

The next bear coming a week or so later collected the remaining eggs, and the third bear in the row grasped the glaucous gull chicks that had been successfully hiding between rocks during the previous raids. The fourth bear checking the island a day later (Figure 2) found a depleted colony without anything edible left.

In all, in that particular year the colony produced only 9 goslings which is approximately 1 % of the potential number. Predation is not that severe in all years, and obviously depends on the date polar bears arrive. In 2007 and 2008 the first bears came when the majority of geese and ducks had hatched the eggs, and only the latest nests were predated.

It needs further investigation to understand why bears arrive early in some years and late in other. In fact, the reason of the recent shift to the coastal area in summer is still unclear. Investigating the consequences of the shift of summer habitat by bears is of particular importance to the population study of barnacle geese of the durch researchers. The offshore islands may lose their importance to colonial breeding birds, possibly pushing geese to alternative breeding sites more inland. It is not clear yet at which scale bear predation takes place but there is little reason to believe that intensive predation is limited to the study area. As offshore islands are among the prime nesting sites to barnacle geese, negative consequences for the whole population might be possible.

(Source: Jouke Prop)

Contact: jouke.prop@wxs.nl

Watch film on YouTube: Polar bear going for the last eggs in the colony. http://www.youtube.com/watch?v=b9UyXSPgPg0

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Figure 1. Number of polar bears observed per 5-day period, Nordenskiöldkysten. The early observations (27 May – 5 June) were done in 1978–1982. All other observations are from 2003–2009. Bears that predated at least 5 nests are indicated in blue. Bears that found less nests or that did not visit the colonies are indicated in red. The bar gives the period when barnacle geese are in the colony.

All phots and figures: J. Prop.