Eating locally: Australasian gannets increase their foraging effort in a restricted range

Abstract

During the breeding season, seabirds adopt a central place foraging strategy and are restricted in their foraging range by the fasting ability of their partner/chick and the cost of commuting between the prey resources and the nest. Because of the spatial and temporal variability of marine ecosystems, individuals must adapt their behaviour to increase foraging success within these constraints. The at-sea movements, foraging behaviour and effort of the Australasian gannet (Morus serrator) was determined over three sequential breeding seasons of apparent differing prey abundance to investigate how the species adapts to inter-annual fluctuations in food availability. GPS and tri-axial accelerometer data loggers were used to compare the degree of annual variation within two stages of breeding (incubation and chick rearing) at a small gannet colony situated between two larger, nearby colonies. Interestingly, neither males nor females increased the total distance travelled or duration of foraging trip in any breeding stage (P>0.05 in all cases) despite apparent low prey availability. However, consistently within each breeding stage, mean vectorial dynamic body acceleration (an index of energy expenditure) was greater in years of poorer breeding success (increased by a factor of three to eight), suggesting birds were working harder within their range. Additionally, both males and females increased the proportion of a foraging trip spent foraging in a poorer year across both breeding stages. Individuals from this colony may be limited in their ability to extend their range in years of low prey availability due to competition from conspecifics in nearby colonies and, consequently, increase foraging effort within this restricted foraging area.

Keywords: Accelerometry; Foraging ecology; Inter-annual; Morus serrator.

Fig. 1.

Comparison of foraging behaviour and effort for Australasian gannets across three years. Behaviours include proportion of foraging trip spent resting on the sea surface (A,B); flapping flight (C,D); gliding flight (E,F); foraging (G,H); and dive rate (dives h⁻¹) (I,J). Energy expenditure is represented by mean vectorial dynamic body acceleration (VeDBA) (g) (K,L). In variables with significant results (P>0.05) homogenous subsets are indicated by superscripts. Data are represented as mean±s.e.m. 2011, blue; 2012, green; 2013, purple. Breeding stages: incubation, circles; chick rearing, squares.

Fig. 2.

Location of Pope's Eye gannet colony. The colony is indicated by the red dot in south-eastern Australia (dark rectangle on inset map A). Environmental data was extracted from (A) Port Phillip Bay and (B) Bass Strait. In addition, environmental data were obtained in the region of the seasonally active (C) Bonney Upwelling (shown in inset map C). Bathymetry and edge of the continental shelf are indicated by light grey lines. Other gannet colonies in the region are indicated by a black dot (proportional to their size, as detailed in the legend). Colony size for Pope's Eye includes all birds nesting in Port Phillip Bay due to their close proximity. As birds typically remain on the continental shelf to forage, arrows indicate potential foraging areas and direction for each gannet colony, based on colony size and location (Wakefield et al., 2013).