It is late June and skua nests should be hatching right now, and one should stumble over chicks on the tundra. Instead pairs are hanging around with nothing else to do than sit at their territory and fly to sea to feed from time to time. Of a total of 40 nests, 33 were predated by red foxes, two more by gulls. In the four years we have worked here so far, we never encountered such high predation rates. One could argue that foxes follow our tracks and we as researchers lead them from nest to nest. Even though we are very careful at nests, our scents will definitely play a role. However also nests that we did not approach, but only observed from a distance disappeared. And all nests disappeared very shortly after they were started. Not only the skuas, but also waders in the area suffer from fox predation. Barbara Ganter and Hans-Ulrich Rösner, who have investigated the Dunlin population here over 25 years, also see more predation than usual.
We visited the active fox den in the area and it made an impression of a lavishly set table: parts of a reindeer calf, remains of several gulls, feathers spread out all over the place. Four nearly full grown young foxes are regularly playing outside the den. A fox den has several entrances and a few were stuffed with even more food: a reindeer calf and a complete cormorant. We guessed the 66 skua eggs were also hidden somewhere inside the den!
And foxes are not the only threat: because of late snow melt the reindeer herd could not feed in the mountains and arrived here extremely early and in much larger numbers than usual. Now 600 reindeer cross the area on a day to day basis. Not only can they trample nests, they will eat eggs if they come across them. Resting herds are often found lying in the middle of known skua territories for hours, preventing them from going back to their nests.
We are working in the Slettnes Arctic skua colony since 2014. That year nests were abundant, skuas were aggressively attacking us, and many chicks survived and fledged. The seasons since then have been worse every year. Less nests, less known birds resighted and hardly any breeding success. The main driving force behind these changes seems to be food availability. Arctic skuas do not feed for themselves, but rely on host species such as Arctic tern and kittiwake. Skuas chase their victims until they throw up the fish they just swallowed. There are no consistent monitoring programs on Arctic skuas, but the kittiwake is in strong decline all along the Norwegian coast (see this NINA report). We counted no more than 80 breeding pairs of Arctic skua in the Slettnes colony last week, while there used to be 250 in 1997/1998. The key food for seabirds, but also for large fish and whales in the area are capelin and herring.
Capelin underwent several collapses and rises since the 1980s. The stock estimate is based on the joint Russian–Norwegian acoustic surveys in September each year (see this report). During winter and early spring Barents Sea capelin migrate to the coast of northern Norway (Finnmark) and the Kola Peninsula (Russia) for spawning. During summer and autumn capelin migrate north- and north-eastward for feeding. The last collapses were caused by poor recruitment, most likely in combination with low growth and increased predation pressure. The 1986 collapse was aggravated by high fishing pressure. Since then fishing pressure has been low. Predation by young herring has been suggested to have strong negative influence on capelin recruitment. The recent strong decline in the capelin stock appears to be caused by a combination of the same factors as in the previous capelin collapses: a decrease in individual growth rate of capelin, increased mortality caused by heavy predation from the large cod stock, and lower recruitment. In 2016 there was better recruitment of both capelin and herring, which could have led to a good food situation in 2017. However we have not witnessed that so far. For bird species relying on pelagic species, also the distribution of the fish is important: it should all be within reasonable flying distance of the colony.
So this is our idea of how the bad food situation has affected the colony in the past few years at Slettnes:
Because of the lack of food there are fewer host birds breeding in the area and feeding out at sea (kittiwakes and Arctic terns)
As a results fewer Arctic skuas start to breed, resulting in a colony with a much looser structure than in good years
Arctic skuas are also less aggressive towards predators because they are in worse condition (the body mass of birds that we captured every year is 5% lower in 2015-2017 than in 2014).
All this gives better opportunities for the Red Fox to enter the colony and predate the skua nests
The skuas also produced smaller and fewer eggs in 2015-2017 than in 2014.
The increasing reindeer herd further aggravates the situation: ca 400 animals wander through the area every day, possibly resulting in trampling of nests (and they will also eat eggs when they come across them).
Of course we have also considered our potential disturbing role as researchers in the area. However, especially in parts of the colony where we did not work, the decline is even more prominent than in the area where we caught most birds and spent most time.
We are back at our scenic study area in Slettnes in the far north of Norway. ‘We’ in this case consist of: Hans Schekkerman, Rinse van Vliet, Marc van Roomen, Janne Schekkerman, Geert Aarts and Ingrid Tulp. Goals for this year is to retrap as many Arctic Skuas and red-necked Phalaropes, with geolocators loaded with data of their whereabouts from the previous year(s), as possible. Last year was extremely early with first skua nests hatching already at 12 June, this year is the opposite. First nests are expected to hatch only at 28th of June. There was an exceptional load of snow in Northern Norway this year, the vegetation is two weeks behind a ‘normal’ schedule. But given that there is enough snow free area at Slettnes, this cannot be the reason for the late start of the skuas. Hopefully the loggers will reveal what kept them so long before arriving here. Up to now we only managed to recapture 4 skuas. We still miss a lot of known birds. Up to now we resighted 37 of the total of 73 birds colour-ringed in the previous years. Many of them still need to start breeding. Yesterday (13 June) we found several one egg nests that just started. All in all we have the impression that breeding numbers are considerably lower here in Slettnes in the period we are coming here (since 2014) than they were in 1998, when the last inventory was carried out. To check this idea, we are going to do a colony wide census this year, something we did not do systematically yet in the recent years.
The red-necked phalaropes are at a more normal time schedule. Groups of males and females are on the lakes, twittering and chasing each other. Hormones are swarming around along the lakes edges! The first birds already have nests. We use a new method to catch the swimming phalaropes: a mistnet held flat between two persons immerged in the water just below the water surface, wait until the birds swim over it and then quickly lift the net from the water and flapping the bird! Yesterday we caught five birds within ten minutes this way. We already managed to retrap three loggered males (of which one unfortunately lost his logger).
An ‘increase year’ was expected for the lemmings and other rodents in Ammarnäs. In such years, rodent populations slowly recover from a crash and build up towards a next peak. At such intermediate rodent densities, Long-tailed Skuas in Ammarnäs usually breed at least in the best area: Raurejaure.
Today we (Morrison Pot and I) crossed the tree line in Ammarnäs for the first time this season. After only about ten minutes the disappointment already sank in: this was going to be another non-breeding year for the Long-tailed Skuas. The second in a row, thereby repeating the pattern of 2012/2013. A pair was sitting quietly, occasionally picking some berries or insects. At this date, one of the partners should be incubating an egg…
The good news is that we resighted several colour-ringed skuas and all were still carrying their geolocators. Two birds have apparently switched partner, meaning two ‘famous’ birds are now missing: KJ, known for its extremely aggressive nest defense, and KM, a bird that we used to see on every visit to Raurejaure, as it bred at the ‘entrance’ of the area.
Another promising observation is that of a male Dunlin carrying a geolocator! Hopefully we will be able to recapture this bird. This bird is part of a big circumpolar effort to track the migrations of all subspecies of Dunlin – there are many!
Today, the 2017 field season started in Slettnes. After arrival, Hans and Rinse already had a first quick look in the colony and resighted some Arctic Skuas carrying geolocators. There seems to be lots of snow up in the mountains – not only in Finnmark but also in Ammarnäs. Morrison and I will not arrive in Ammarnäs until next week, so hopefully the snow will melt quickly in the coming days…
Our main aim is to recapture Arctic and Long-tailed Skuas, Red-necked Phalaropes and also Dunlins carrying geolocators so we can map their movement since the last year. Whether we can retrap them depends on whether they return to the same spot and whether or not they breed this year. We will know soon!
We are very happy to share with you the publication of our last paper: “Habitat selection, diet and food availability of European Golden Plover Pluvialis apricaria chicks in Swedish Lapland”. It has been a long way but it is finally here!! Click here!
This study shows the prey preferences of Golden plover chicks in the alpine tundra in northern Sweden, as well as their habitat use and food availability. Interestingly, a peak of bibios (Bibionidae, also known as marsh flies) at the end of the season in 2011, made the chicks change their diet to practically only feed on them, so we highlight the importance of this seasonal effects on the food availability, ergo diet, survival and growth of the chicks. We also show that plovers in Lapland do not have Tipulids (Crane Flies) as their favorite prey, as it happens to be in another population (UK), where chicks rely on them.
We are recently working on the second part of this story, analysing which factors influence the growth of the chicks. Hope to show it to you soon!
One of the aspects of migration that continues to fascinate people, is the ability of individuals to return to the same spot (either a wintering or breeding site), year after year. Such site-faithful individuals can be relatively easy to record, but recording site INfidelity has always been a major difficulty: if a bird doesn’t turn up where it was last year, you didn’t detect it, or it either moved or died. Tracking studies do not suffer from these problems, and now that tracking studies are producing data from individuals over multiple years, we can finally study the degree of consistency in routes and site selection over the entire annual cycle.
Recently, we published a paper reporting on this subject in Long-tailed Skuas. The paper is published in a theme section on ‘individual variation in migration and foraging in seabirds’ of the journal Marine Ecology Progress Series and can be accessed here. Don’t forget to click on the supplement, which contains a nice animation of the tracks!
Over half of the geolocator data used for this study was obtained from the Long-tailed Skuas in Ammarnäs, with the other part coming from colleagues working in Greenland and Svalbard. Together, we mapped nearly a hundred annual migrations of 38 individual Long-tailed Skuas. Several individuals were tracked over four or five years.
The emerging pattern is remarkable. While Long-tailed Skuas are generally faithful to their route from the previous year, they occasionally choose to go somewhere completely different. The highest consistency is found in early winter, around the time when they arrive at the main wintering area. After mid-winter, individuals start to deviate from their previous route. Such deviations can be huge: one individual went to the Benguela Current in four years, but after arriving in the second and fourth year, it simply flew 5200 km across the Atlantic to spend the remainder of the winter in the Falkland Current, near Argentina! In both occasions it used virtually the same route to get there and ended up in the same area. These patterns suggest they first check out local conditions before deciding where to go subsequently: either continue on their usual route, or on some alternative.
We also showed that individuals can show both ends of the spectrum: individuals showing ‘flexibility’ (deviations from their previous route), are not necessarily always flexible, they can also follow the same route twice.