Foraging ecology and reproductive performance of sympatrically breeding larid species at a North Sea colony

• Main Author: Robertson, Gail Sheila
• Format: Thesis
• Language: English
• Published: 2015
• Subjects: QL Zoology; Arctic; rissa tridactyla; Sterna paradisaea
• Online Access: http://theses.gla.ac.uk/6149/; http://theses.gla.ac.uk/6149/1/coversheet.pdf; http://theses.gla.ac.uk/6149/2/2014RobertsonPhD.pdf; https://eleanor.lib.gla.ac.uk/record=b3099259

Marine ecosystems provide essential goods and services to human populations, however anthropogenic offshore activities can adversely affect the functioning of ecosystems by reducing biodiversity. Temporal data on environmental fluctuations are required in order to implement effective ecosystem management. The health of marine ecosystems can be assessed using proximal measurements of biological data such as fishery catch statistics. However, these data are often patchily distributed and underreported. Seabirds have been shown to be useful indicators of the state of the marine environment. They are highly visible, charismatic species that are easy to count and observe in colonies and offshore foraging areas during the breeding season. In recent years the miniaturisation of electronic devices and the development of novel tracking methods have allowed a large variety of seabird species to be tracked to and from foraging areas and for environmental conditions in distant pelagic areas to be sampled. It has been suggested that seabird foraging and breeding behaviour provide more accurate measures of environmental change than demographic parameters such as adult mortality and productivity, as many species are able to buffer the effects of low food abundance during the breeding season by increasing foraging effort. In this thesis, demographic and behavioural data of several sympatrically breeding larid species were examined over extended temporal scales and the effectiveness of these data at indicating environmental change are assessed. Comparisons of annual fluctuations in demographic parameters were made among ecologically similar and dissimilar tern species breeding sympatrically at a North Sea colony. Species with similar foraging and breeding behaviour exhibited synchronous temporal population fluctuations, while dissimilar species showed no synchrony in population change. Similar and dissimilar species also showed differing responses to declines in predator abundance. To understand how seabird species with similar ecological requirements are able to coexist in the same area during the breeding season, foraging behaviour and reproductive parameters were examined among three morphologically similar terns (Sterna spp) breeding at the same colony. Species partitioned resources by both chick diet and foraging area and responded differently to increasing brood age. Sympatrically breeding Arctic (Sterna paradisaea) and Common Terns (S. hirundo) maintained comparable growth and survival rates of chicks, which suggests that species utilising different foraging strategies can be equally successful at raising chicks to fledging. However, Common Terns were found to exploit larger prey items with higher energetic contents than Arctic Terns, which may explain why this species was able to lay larger clutches and fledge more chicks. Temporal variation in foraging behaviour was examined further in Black-legged Kittiwakes (Rissa tridactyla) by comparing the foraging behaviour of individuals at the same colony during two stages of the breeding cycle (incubation and chick-rearing) and in two consecutive years (2011 and 2012). Diet, foraging areas and environmental variables associated with foraging were found to vary significantly throughout the breeding season and between years, with important consequences for marine conservation policy. Parental resource allocation in Kittiwake broods of two also varied throughout the chick development period as chick demand and environmental conditions surrounding the colony changed. This research illustrates how long-term seabird population and foraging behaviour data can be used to examine changes in the marine environment and to address ecological questions. Variation in chick demand, environmental conditions and species interactions can explain temporal changes in the foraging behaviour of sympatrically breeding species. These results also illustrate the value of considering seabird foraging behaviour when developing effectual offshore protected areas for marine predators.