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 monitoring multiple populations of a species can be investigated to identify potential drivers of population changes. However, as many apex predators are long-lived with a low reproductive output there can be a time-lag before detecting changes in population abundance reflecting adverse environmental conditions. Therefore, instead traits which reflect environmental conditions during the breeding season may be useful in monitoring habitats over shorter time frames. Of several widespread seabird species associated with the coastal environment across the region of interest of south-west Scotland and Northern Ireland, 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 investigate several herring gull traits from multiple colonies to determine whether these alternative traits can effectively be used to monitor the coastal marine environment. In particular I am interested in three features of these traits: (i) do they show spatial variation in the selected colonies across the region of interest; (ii) are these traits sensitive to local environmental conditions reflecting local resource availability and (iii) do the traits relate to a short- and long-term measure of the gulls' demography. Spatial variation between colonies was observed in all selected traits associated with eggs, the extent of marine and terrestrial resources consumed and with the gulls' foraging behaviour; albeit to differing extents. Firstly, I investigate the resource use of herring gulls as food resources are often an important limiting factor in seabird colonies. The resource use of gulls was estimated via two techniques, pellets and stable isotope analysis of chick feathers, which gave comparable results. Colonies located in sheltered coastlines, with more favourable intertidal habitats, were found to forage more on marine items; whilst colonies located nearer built-up areas foraged to a greater extent on terrestrial items. 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. Traits associated with the herring gull's eggs showed different sensitivities to local conditions. Both egg colour and volume were found to be influenced by the ambient temperature prior to laying; with larger eggs and less brown eggs in colonies associated with higher ambient temperatures. In colonies with higher local SST and chlorophyll a concentrations, eggs were found to have greater maculation. Egg colour was also found to relate to the gulls' overall breeding success in 2014, with higher final brood sizes in colonies where eggs were less brown in colouration. Finally, investigating the gulls' foraging behaviours, nest attendance was higher in colonies with a higher amount of built-up area within the foraging range; and, during 2013, in colonies closer to farmland. In colonies with higher local SST and chlorophyll a concentrations, provisioning rates were higher and trip durations shorter; whilst provisioning rates were also higher in colonies located in areas of high wave fetch, along more exposed coastlines. Of the foraging behaviours trip duration was found to relate to the gulls overall breeding success; with shorter trip durations in colonies with higher breeding success. These results highlight that herring gull traits are sensitive to local environmental conditions during the breeding season, especially egg maculation, the extent of marine resources the gulls consume and the three foraging behaviours; and therefore may be effective in monitoring the coastal environment. In addition, it suggests that average egg colour, the extent of marine resources the gulls consume and trip duration of a colony, are useful in reflecting environmental conditions as well as reflecting the gulls' demography relating in breeding success. Investigating herring gull traits from multiple colonies, which reflect environmental conditions over shorter time periods than colony GR, may therefore be useful in monitoring the coastal environment. In addition, they may help establish why this species has experienced contrasting colony GR in recent years; leading it to be categorised as a red-listed bird on conservation concern in the UK.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:702345 |
| Date | January 2016 |
| Creators | O'Hanlon, Nina Jayne |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/7860/ |
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