Across landscapes and seascapes : The movement ecology of diving and flying guillemots and gulls during breeding

Sammanfattning: Some seabirds we encounter only fleetingly, as they fly by as we walk along the beach, or perhaps meet far out at sea when we take a boat trip. Others though we are more familiar with, such as the ‘seagulls’ that many will not even turn their head to appreciate. Yet, all seabirds have remarkable stories to tell, and we are fortunate now to live at a time where we can follow individual birds as they fly out to sea, and even as they dive down under the sea. So they can now tell their own stories through their movements. This is all thanks to small devices sharing much in common with the smartphone in your pocket. In this thesis I follow several species of seabird during the spring and early summertime, when they come to land to lay their eggs and raise chicks. I look at how they forage during this time and how they react and respond to the changing conditions around them both across the season, within days, and as the winds change. In this thesis I study five species of seabird. The Common Guillemot (or Murre in North America, and the internationally accepted common name, hence Murre is used throughout the rest of this thesis), effectively a rather small flying penguin, the guillemots can fly out from the rock ledges on which they breed on sea cliffs where they lay their single egg and raise their chick, then land on the sea where they then proceed to dive deep down under the water (85 m is the record in this thesis, but they have been recorded to dive >200 m) to catch fish. On the same Swedish island as the guillemot, at Stora Karlsö, located in the western central Baltic Sea, I also followed Lesser Black-backed Gull. These make a nest where they raise up to three chicks. Like the guillemots, these gulls can feed out at sea, but they only catch prey at the surface, unlike the guillemots they also forage inland. The final three species are three further gull species, these studied further north, in eastern central Sweden, at the Björn Archipelago at Fågelsundet south of Gävle. The four gull species together range in body size from the small Common (or Mew) Gull, through to the Lesser Black-backed Gull, the Herring Gull, and up to the largest of all gulls, the Greater Black-backed Gull. I used GPS devices to learn where the birds went and when. The GPS gives us more than just a location, it also tells us how fast, in what direction, and how high our birds fly. The GPS is a small match-box sized device that sits on the back of our birds, either attached by tape to feathers (on guillemots) or by a backpack type harness (the gulls). We learn how the guillemot dives thanks to water pressure increasing predictably with depth, thus by recording the pressure experienced by the guillemot over time we can observe their diving activity; this I did using a second device, a time-depth recorder or ‘TDR’. The TDRs are much smaller than the GPS, and were attached on plastic rings to the guillemots’ legs. In paper I, I found that the guillemots mostly forage around sunset and sunrise, at these times they make more frequent and shallow dives compared to around midday. This likely reflects that the guillemots need light to forage, so can only forage deep down under the midday sun. More importantly though, maybe, the fish they eat spend most of the day in deeper water, but at night these come close the surface where they forage on smaller animals (zooplankton), thus the fish are then more assessable to the guillemots. The guillemots made two types of foraging trip, longer duration and distance overnight trips, and shorter daytime trips. On the long overnight trips they travelled further, potentially to more productive areas with more fish available, on the shorter trips they foraged relatively near to their nesting island. This may allow them to balance feeding for themselves (long trips) with catching fish for their chicks (short trips). We assume that the stories told by the seabirds through their movements recorded with the electronic devices do not differ from that told by birds not carrying devices; in other words we assume the devices do not much affect the activity of the birds tracked. Following up from paper I, I set out to investigate this in paper II. I compared the activity of guillemots carrying both the GPS and TDR devices, and the same individuals with just the TDR. In this way I found that their foraging activity was little affected by carrying the GPS devices. However, whilst carrying GPS loggers, the guillemots lost more mass than when without, suggesting some effect of loggers on the guillemots’ condition. This could suggest that the guillemots used more energy to maintain the same activity levels, or that the extra weight and drag from the device reduced their success in catching their prey fish. For birds flying through the air, wind has a large influence on how quickly they can travel, and how much energy they must use to travel, it can be both beneficial and a negative influence depending on the direction of the wind relative to where the bird wishes to travel. Therefore, we expect birds to change their flight behaviour as wind conditions change. This I investigated in paper III. During their flights returning from foraging sites out at sea back to their colony at Stora Karlsö, both the guillemots and gulls adjusted their speed of flight relative to the air around them (airspeed) to minimise their energy use for a given distance travelled. They increased their airspeed under headwinds and cross-winds, but reduced their airspeed under tailwinds. While the gulls also adjusted how high they flew, flying higher under tailwinds, thus benefiting from the faster winds higher up, but flying low, where wind is slower, under headwinds and crosswinds. The guillemots always flew low, close to the sea surface. Gulls are flexible foragers, able to use a wide range of prey found in different habitats, both on land and at sea. I investigate how they vary their foraging activity across habitats, over time, and between species in papers IV and V. The Lesser Black-backed gulls at Stora Karlsö feed both out at sea and on arable fields on nearby Gotland. In paper IV, I followed individual gulls with GPS and complemented this with observations on the arable fields they visit. The gulls foraged most on land early in the breeding season, and during early mornings especially on cool and cloudy days, while they foraged mostly at sea later in the season, and later in the day. When on land, they foraged most on fields with short vegetation and low ground-cover. Together this suggests the gulls are feeding on soil living invertebrates and earthworms especially; for earthworms come more to the soil surface at night (continuing into the early morning) especially under damp conditions. The seasonal changes may either come about through the gulls having different food requirements through the season, for themselves or their eggs/chicks, or through changes in the availability of different prey. The study suggests that availability may play a role, for the most suitable arable fields (short vegetation with bare soil) were more available early in the season. At the Björn Archipelago, at Fågelsundet, five different species of gull breed alongside each other across a range of small islands. In paper V, I followed four of these species with GPS to compare how they each forage in this common area. I got data for nearly 800 foraging trips made by these species. To find out whether individual gulls across species used similar types of foraging trip I used an analytical method where similar trips were grouped together, clustering. This suggested seven main types of foraging trip used across all the species. Some of these were used by multiple species, such as foraging on landfill/refuse sites, which both Herring and Great Black-backed gulls did. While, others were nearly unique to one species, this was the case for long distance foraging trips travelling out to sea, used nearly exclusively by the Lesser Black-backed gulls. Species also varied in how many different types of trip they performed, with Common Gull using mostly five different types of trip, whereas the Lesser Black-backed gulls mostly used just two types of trip. Patterns also varied within species, though overall a species might use several different foraging trips types, any given individual within the species might only use a subset of these. This was the case for the Great Black-backed gulls. Conversely, by following just one Herring Gull we would observe the majority of the types of trips this species uses, for all individuals were similar in their foraging patterns. In the final study, paper VI, we leave the immediate area around the study islands, as Lesser Black-backed gulls travel widely across the Baltic Sea. Following breeding failure, or under poor local conditions birds may choose to move to a new breeding site. They could do this through random dispersal, but more effectively they may visit a range of potential sites to find which are best. Following breeding failure, many of the Lesser Black-backed gulls followed by GPS at both study sites made long distance return movements lasting up to 27 days. On some trips they remained at sea, visiting more distant, but likely more productive, foraging areas. On the majority though they would visit multiple coastal sites around the Baltic Sea islands and coasts, this suggests prospecting at alternative breeding colonies. The gulls from Björn Archipelago travelled furthest, both to the north and visiting many of the same coastal sites as the Stora Karlsö gulls across the south-central Baltic Sea.