Population trends of breeding birds along the Swedish coast

Abstract

I present here population trends of 52 bird species with coastal- breeding populations for the period 1990 to 2016. The trends are based on data collated from 51 surveillance programmes on breeding birds, which together covered most of the Swedish coastline.
I related the trends to foraging guild, the species' distribution, and the species' climatic envelope. I also compared trends inside and outside the network of Natura 2000 Special Protected Areas (SPAs). Countrywide, fourteen species increased in numbers and fourteen declined, while twelve showed stable trends and the remaining twelve uncertain trends. I subdivided the Swedish coast into north (the Gulf of Bothnia), south (the Baltic Proper) and the West Coast to check for regional variation in trends. The trends for diving ducks Aythini/Mergini and gulls Larus sp. were consistently worse in the south than in the north, a pattern also found for several other species. Trends in the south were significantly lower (decline) than in the north and west, signalling a northward shift in breeding distributions on the East Coast. Terns Sterninae and dabbling ducks Anas/Mareca were however increasing or stable in numbers over all regions. Roughly speaking, herbivorous and warm-adapted species increased while wading and benthic feeders – as well as cold-adapted species – declined.
Distance between breeding site and the mainland could explain variation in yearly indices for all the fourteen most commonly recorded species. For ten out of those species (mallard Anas platyrhynchos, tufted duck Aythya fuligula, oystercatcher Haematopus ostralegus, mew gull Larus canus, herring gull Larus argentatus, velvet scoter Melanitta fusca, red-breasted merganser Mergus serrator, goosander Mergus merganser, eider Somateria mollissima and common tern Sterna hirundo), the part of the population that bred on sites less than 1 km from the mainland had more positive trends than those breeding further offshore. I suggest that this could be due to predator avoidance behaviour, especially avoidance of white-tailed sea eagle Haliaeetus albicilla and possibly also North American mink Neovison vison.
Lastly, Caspian tern Hydroprogne caspia was found to have a larger population increase inside SPAs than on sites outside of the network during 1990- 2016. For other species, the effect of SPAs could not be disentangled from the effect that distance from the mainland had on the trends.

Popular Abstract

How are Sweden's coastal birds doing?

How are Sweden’s coastal birds doing? To answer this question, I studied at a dataset containing observations from over fifty different local projects that together covered almost all of the Swedish coast. The projects entailed visiting at least one breeding location (e.g. an island/skerry/group of islands) at least one year between 1990 and 2016 to count birds in the summer. Ideally, all locations would have been visited every summer, – this way the exact population development on every single location would be known. But this was of course not the case. By using statistics I predicted missing counts based on observations, and found the coastal population trend of 52 species during parts of or the whole period 1990 to 2016. The results were mixed: fourteen species increased, fourteen species declined, twelve species had stable numbers, and the rest had an uncertain trend. Ducks, swans and geese, i.e. species who feed on plants on land and in the water, had the most favourable population trends. The barnacle goose Branta leucopsis was for instance the species with the strongest increase of all 52, with a 26% yearly increase in the population between 1991 and 2016. On the other hand, none of the diving duck or shorebird species increased. For instance eider Somateria mollossima – one of the most common species in the Baltic Sea – declined by an average of 5% per year 1990- 2016.

To investigate the different trends closer, I looked at regional subsets of the data for each species. These regions were the northern Baltic Sea, the southern Baltic Sea and the West Coast. I found that the trends in the southern Baltic Sea were worse than in the other regions for most species. For instance, the diving ducks (eider, tufted duck Aythya fuligula and velvet scoter Melanitta fusca) were all declining in the south, but stable or increasing in the north of the Baltic Sea. However, the terns and the plant-eating birds had similar trends in all regions. The regional differences could mean that the species had shifted to breed further north as a response to climate change and/or other changes in the Baltic Proper. I also found a correlation that warm- adapted species (for instance gadwall Mareca strepera) increased, while cold-adapted species (for instance velvet scoter) decreased in Sweden since 1990.

I investigated if the Natura 2000 network of Special Protected Areas (SPAs) benefited the breeding populations of threatened species. For Caspian tern Hydroprogne caspia the population did indeed increase more in locations inside SPAs than outside of the network. The distance between a breeding site and the mainland also had a significant effect on the trends of the most commonly recorded species. Trends were on average better in locations less than one kilometre from the mainland or the big islands (Gotland and Öland), than further offshore. One explanation could be that by breeding closer to the mainland ¬i.e. closer to human settlements, species get protection from predators like white-tailed sea eagle Haliaeetus albicillia and mink Neovison vison.


Master’s Degree Project in Biology 30 credits 2017
Department of Biology, Lund University

Advisor: Fredrik Haas
Biodiversity Unit, Department of biology

• author: Lund Bjørnås, Kristine
• supervisor: Fredrik Haas
• organization: Degree Projects in Biology
• course: BIOM01 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8925007
• date added to LUP: 2017-09-07 16:43:27
• date last changed: 2017-09-07 16:43:27