BRANTA — Nina Jayne O'Hanlon
Spatial variation in Herring Gull traits and their potential as monitors of the coastal environment
Institution: University of Glasgow, UK
Supervisors: Ruedi Nager
Details: PhD, 2016
31 Bridgewater Drive
Subject Keywords: Species Keywords: Herring gull, Larus argentatus
Coastal marine environments are important for biodiversity and ecosystem services; however human pressure on coastal ecosystems has increased markedly over the last several decades. In order to determine the state of coastal environments apex predators, in particularly seabirds, can be used to monitor these habitats. At the population level, multiple populations of a species can be monitored to identify potential drivers of population changes. However, as many apex predators are long-lived, with low reproductive rates, there can be a time-lag before changes in population abundances, reflecting adverse environmental conditions, are detected. Instead traits that reflect environmental conditions during the breeding season may be useful in monitoring coastal habitats over shorter time frames. The aim of this study was to establish whether spatial variation in seabird traits, reflecting environmental conditions over different time scales, could provide information on the state of the local coastal environment. Of several widespread seabird species associated with the coastal environment across south-west Scotland and Northern Ireland, the region of interest, the herring gull Larus argentatus was found to show the greatest potential as a monitoring species. Spatial variation in herring gull colony growth rate was observed across the region, potentially driven by the availability of local intertidal and terrestrial resources. I therefore investigated several herring gull traits from multiple colonies with contrasting population trends. Traits were associated with the herring gulls’ eggs, resource use and foraging behaviour. In particular I was interested in three features of these traits: (i) was there between-colony spatial variation in the selected traits; (ii) were the traits sensitive to local environmental conditions reflecting local resource availability and (iii) whether the traits related to a short- and long-term measure of the gulls’ demography. Spatial variation between colonies was found to occur in egg colour, maculation and volume; the extent of marine and terrestrial resources consumed; and with the gulls’ foraging behaviour, albeit to differing extents. I then investigated which environmental variables might be driving these differences, specifically focusing on food resources, which is often an important limiting factor in seabirds. The resource use of herring gulls was estimated via two techniques, pellets and stable isotope analysis of chick feathers, which gave comparable results. Colonies located in sheltered coastlines, with availability of intertidal prey, were found to forage more on marine items; whilst colonies located nearer built-up areas foraged to a greater extent on terrestrial items. Traits associated with the herring gull’s eggs showed different sensitivities to local conditions. All three egg traits that showed significant spatial variation between colonies (colour, maculation and volume) were influenced by the minimum ambient temperature prior to laying, whilst egg maculation and volume were also influenced by daily precipitation levels. In colonies with a large amount of farmland within the foraging range of the colony eggs were larger and more maculated, suggesting that this habitat was important for the gulls. Egg maculation was also greater in colonies located in more sheltered coastlines, associated with a higher availability of intertidal prey. Finally, investigating the gulls’ foraging behaviours, nest attendance was higher in colonies closer to farmland, whilst provisioning rates were higher in areas of high wave fetch, along more exposed coastlines, and with little or no built-up area within the foraging range of the colony. These results suggest the importance of both terrestrial and marine resources to the gulls during the breeding season. Where herring traits showed spatial variation related to variation in local environmental conditions it might be expected that this would be reflected in measures of the gulls’ demography. For resources use I did find that the resources the gulls predominantly consumed were found to influence their demography, with greater breeding success in colonies which consumed a higher proportion of marine resources. However, I found no relationships between egg traits or foraging behaviours on the herring gulls breeding success. In addition, none of the herring gull traits were related to the long-term measure of demography, colony GR. My findings indicate that the resources the gulls consume, which is related to what is available within the vicinity of the breeding colony, is the most influential factor affecting the birds during the breeding season. Although both marine and terrestrial resources appear to be important, foraging on marine resources benefited the gulls to a greater extent in terms of breeding success. These results highlight that herring gull traits are sensitive to local environmental conditions during the breeding season, particularly egg volume, the extent of marine resources the gulls consume and nest attendance. Therefore, herring gull traits do have the potential to be effective in monitoring the coastal environment. In addition, the extent of marine resources the gulls consume may be useful in reflecting environmental conditions as well as reflecting the gulls’ demography relating to breeding success. Investigating herring gull traits from multiple colonies, which reflect environmental conditions over shorter time periods than colony GR does have potential in providing information on the local coastal environment, however the mechanisms involve are not always clear. In addition, these results may help establish why this species has experienced contrasting colony GR in recent years, leading it to be categorised as a red-listed bird of conservation concern in the UK.