Sexual segregation in a wide-ranging marine predator is a consequence of habitat selection

Abstract

Sexual segregation, common in many species, is usually attributed to intra-specific competition or habitat choice. However, few studies have simultaneously quantified sex-specific foraging behaviour and habitat use. We combined movement, diving, stable isotope and oceanographic data to test whether sexual segregation in northern gannets Morus bassanus results from sex-specific habitat use. Breeding birds foraging in a seasonally stratified shelf sea were tracked over 3 consecutive breeding seasons (2010−2012). Females made longer trips, foraged farther offshore and had lower δ ) and sea-surface temperature (SST) was relatively low (<10°C). Males also tended to use areas with higher SSTs (>15°C) more than females, possibly as a consequence of foraging in productive mixed waters over offshore banks. Females foraged most frequently in stratified offshore waters, of intermediate SST (12−15°C), but exhibited no consistent response to NPP. Sex-specific differences in diving behaviour corresponded with differences in habitat use: males made more long and deep Ushaped dives. Such dives were characteristic of inshore foraging, whereas shorter and shallower V-shaped dives occurred more often in offshore waters. Heavier birds attained greater depths during V-shaped dives, but even when controlling for body mass, females made deeper V-shaped dives than males. Together, Male gannet Morus bassanus about to depart on a foraging trip.Photo: Keith Hamer these results indicate that sexual segregation in gannets is driven largely by habitat segregation between mixed and stratified waters, which in turn results in sex-specific foraging behaviour and dive depths.

Sex-specific macronutrient foraging strategies in a highly successful marine predator: the Australasian gannet

“…Our results suggest that males and females bring nutritionally imbalanced foods to the nest, but when combined allow the offspring to reach its multidimensional nutritional target of protein, lipid, water and micronutrients (Montevecchi and Porter 1980;Morehouse et al 2010). In this case, adults' nutritional needs are likely to influence foraging behaviour (diving, foraging trip durations and habitat use), such that different sexes will exploit vertical and horizontal habitats to capture certain foods that enable them to attain specific nutrients (Lewis et al 2002;Cleasby et al 2015). Gannets have characteristic pair-greeting ceremony behaviour as part of the changing guard at the nest, known as bill fencing (Nelson 1978).…”

“…These long-lived marine predators with biparental care have been conventionally considered monomorphic species, but a recent study suggest morphometric differences between sexes in two Australian colonies (Angel et al 2015). In Northern gannets (M. bassanus), sex-specific differences have been observed in habitat use (Lewis et al 2004;Cleasby et al 2015), diving behaviour (Lewis et al 2002) and prey consumption (Stauss et al 2012). In Cape gannets (M. capensis), asymmetries between the sexes have been noted in reproductive investment (Bijleveld and Mullers 2009), foraging distance and foraging duration (Mullers and Navarro 2010).…”

Individual-level Variation and Higher-level Interpretations of Space Use in Wide-ranging Species: An Albatross Case Study of Sampling Effects

“…The variability among sub-samples should be assessed including maximum and minimum estimates in area occupied, along with the range in geographic locations of those areas. Increasingly, studies are including significance tests for overlap analyses; the proportional area of overlap between specified contours estimated for groups of interest from full datasets are compared with those estimated from randomized iterative sub-samples as a test of whether enough individuals were tracked to make reasonable higher-level inferences of significant spatial segregation (e.g., Breed et al, 2006;Kappes et al, 2011;Cleasby et al, 2015;Orben et al, 2015). This approach, coupled with area saturation curves, can improve confidence in the appropriateness of higher-level extrapolations.…”

Sex‐specific prey partitioning in breeding piscivorous birds examined via a novel, noninvasive approach

“…For seabirds, such variations in feeding and behavioral ecology have been frequently observed (e.g., Bearhop et al., ; Widmann et al., ), and they can strongly influence predation pressure exerted on prey populations and interfere with management and conservation issues (Phillips, McGill, Dawson, & Bearhop, ; Thalmann, Baker, Hindell, Double, & Gales, ). However, investigations on sex‐specific prey choice and potential resource partitioning of breeding piscivorous birds so far remain mostly limited to marine environments (e.g., Cleasby et al., ; Ismar, Raubenheimer, Bury, Millar, & Hauber, ; Robinson, Forbes, & Hebert, ).…”

“…Baker, Hindell, Double, & Gales, 2007). However, investigations on sex-specific prey choice and potential resource partitioning of breeding piscivorous birds so far remain mostly limited to marine environments (e.g., Cleasby et al, 2015;Ismar, Raubenheimer, Bury, Millar, & Hauber, 2017;Robinson, Forbes, & Hebert, 2009).…”