Besel, Diana (2017) Inter-annual plasticity of foraging patterns in a sexually monomorphic seabird species, the Australasian gannet (Morus serrator). (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 55 pp.

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Abstract

Variations in foraging trip characteristics of seabirds are apparent during different breeding stages, with varying brood sizes or between years with changing environmental conditions. Furthermore, male and female birds of the same species can display differences in foraging behaviour, which has been observed in size dimorphic as well as in monomorphic species. While foraging trip characteristics have been extensively studied in the Northern gannet (Morus bassanus), data for the related Australasian gannet is quite rare. In this study I wanted to analyse foraging trip parameters of the Australasian gannet, a largely size monomorphic seabird species, on pre-collected foraging tracks at a major New Zealand mainland breeding site for the species. Specifically I aimed to assess differences in trip parameters and core foraging areas between males and females, variations between the different breeding stages and three consecutive breeding seasons. I did not find any significant differences for the trip parameters analysed between male and female Australasian gannets. Foraging trips were 33.19 km longer during chick provisioning compared to incubation. In addition, foraging trips were more than twice as long in terms of duration in 2009-10 compared to the other two seasons, and in 2007-08 gannets spent significantly more time at sea than in 2008-09. There was no significant difference in the size of the core foraging areas used between the sexes, the breeding stages or the three breeding seasons. However, there was spatial sexual segregation of core foraging areas with little or no overlap between areas of males and females during each of the three breeding seasons, and the position of the foraging areas of each sex differed between the seasons, as well. Furthermore, gannets used a very different core foraging area in 2009-10 than in the other two seasons. In the 2007-08 and 2009-10 breeding season gannets used larger home ranges during chick provisioning than incubation, while in 2008-09 the reverse pattern could be observed. Besides, I found evidence for a bimodal foraging strategy in Australasian gannets in two of the three breeding seasons, with trips being either less than 15 h or more than 20 h in duration. These results suggest high inter-annual variability in foraging behaviour of Australasian gannets and underline the necessity for future studies to incorporate data from more than one breeding season.

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