The Causes and Consequences of Individual Over-Wintering Strategies in Northern Gannets
Williams, Hannah Jane
Thesis or dissertation
University of Exeter
Within populations, individual behavioural variation is determined by intrinsic factors such as morphological sex differences, or extrinsic environmental factors. However, accounting for these differences, individual variation often persists as a result of individual niche specialisation. Variation in the foraging behaviour of avian marine predators has received particular attention, owing to the strong association between these species and fishing vessels. While the foraging strategies of seabirds have been investigated in the breeding season, non-breeding strategies and their carry-over effects (COEs), remain poorly understood. Furthermore, behaviour in one season should be considered in the context of previous and subsequent processes, as there are likely downstream impacts between seasons. In this thesis I address this gap in our understanding by investigating individual variation in the over-winter behaviour of Northern gannets (Morus bassanus) from a number of UK breeding colonies. Two types of foraging behaviour have been described for diving seabirds: natural plunge-diving and discard foraging, the prevalence of which suggests population and individual flexibility in foraging strategy. Furthermore, with changes in discard availability likely to occur with future fisheries management, it is important to understand the predictors of this foraging behaviour and COEs on subsequent breeding success. I combine sequential tissue sampling for stable isotope analysis with geolocation to quantify individual repeatability in diet and hence foraging strategy, and parameters of migration. My findings reveal, firstly, that the UK gannet population is flexible in its foraging behaviour, as individuals exhibit one of three foraging strategies differing in trophic level and foraging habitat specialisation (Chapter 2). One of these strategies suggests a high level of discard foraging. Secondly, I explore previous individual state as a predictor of foraging behaviour and the destination of winter migration. Here I find that while migration strategy appears to be limited by state, foraging strategy is a result of individual niche specialisation independent of state. However, individuals of poor state foraged from a high trophic level, suggestive of discard foraging. Finally, in Chapter 3, I quantify the consequences of over-wintering behaviour on individual arrival at the breeding colony and subsequent onset of breeding. Perhaps surprisingly, early arrival does not correspond to earlier breeding. However, I find the telling result that individuals foraging, on what is suggested as discards, hatch their chicks later in the season, which may be attributable to the low nutritional value of such a diet. I conclude firstly that migration appears to be limited by previous state. And secondly, that foraging from a high-trophic level may enable individuals of low state to survive, but with this strategy comes a cost for subsequent breeding. The level of detail achieved in following this large set of individuals over multiple years, reveals the interaction between individual niche specialisation and carry-over effects, and highlights the continuous impacts of discard foraging in a population so closely associated with anthropogenic fishing practices.
MbyRes in Biosciences