A review of the occurrence of inter‐colony segregation of seabird foraging areas and the implications for marine environmental impact assessment

Abstract

Understanding the determinants of species' distributions is a fundamental aim in ecology and a prerequisite for conservation but is particularly challenging in the marine environment. Advances in bio-logging technology have resulted in a rapid increase in studies of seabird movement and distribution in recent years. Multi-colony studies examining the effects of intra-and inter-colony competition on distribution have found that several species exhibit inter-colony segregation of foraging areas, rather than overlapping distributions. These findings are timely given the increasing rate of human exploitation of marine resources and the need to make robust assessments of likely impacts of proposed marine developments on biodiversity. Here we review the occurrence of foraging area segregation reported by published tracking studies in relation to the density-dependent hinterland (DDH) model, which predicts that segregation occurs in response to inter-colony competition, itself a function of colony size, distance from the colony and prey distribution. We found that inter-colony foraging area segregation occurred in 79% of 39 studies. The frequency of occurrence was similar across the four seabird orders for which data were available, and included species with both smaller (10-100 km) and larger (100-1000 km) foraging ranges. Many predictions of the DDH model were confirmed, with examples of segregation in response to high levels of inter-colony competition related to colony size and proximity, and enclosed landform restricting the extent of available habitat. Moreover, as predicted by the DDH model, inter-colony overlap tended to occur where birds aggregated in highly productive areas, often remote from all colonies. The apparent prevalence of inter-colony foraging segregation has important implications for assessment of impacts of marine development on protected seabird colonies. If a development area is accessible from multiple colonies, it may impact those colonies much more asymmetrically than previously supposed. Current impact assessment approaches that do not consider spatial inter-colony segregation will therefore be subject to error. We recommend the collection of tracking data from multiple colonies and modelling of inter-colony interactions to predict colony-specific distributions.

At-sea movements of wedge-tailed shearwaters during and outside the breeding season from four colonies in New Caledonia

“…Birds from these colonies spent most of their time foraging over shallower waters during short trips, but over deep waters during longer trips. In seabirds, segregation between colonies is a widespread phenomenon, and overlap occurs generally in highly productive areas (Bolton et al 2019). Most studies reported in the review of Bolton et al (2019) concern non-tropical waters.…”

“…In seabirds, segregation between colonies is a widespread phenomenon, and overlap occurs generally in highly productive areas (Bolton et al 2019). Most studies reported in the review of Bolton et al (2019) concern non-tropical waters. Comparatively few studies exist for tropical seabirds.…”

“…The presence of this very large population of WTS located in low-productivity waters may entail a potentially high level of intraspecific competition among individuals, conditions that can lead to specific foraging behaviours that reduce competition, such as longer foraging ranges (Ashmole halo effects; Ashmole 1963), or to the separation of foraging zones between colonies (Lewis et al 2001). Segregation between colonies, considered to result from inter-colony competition, is itself a function of colony size, foraging range and productivity (Bolton et al 2019). Thus, studying geographic overlap in foraging areas between colonies within a large population that relies on poor surrounding tropical waters is of particular interest when considering the evolution of segregation.…”

“…Here, we studied foraging behaviour during both the breeding and the migratory periods in WTS from 4 colonies in New Caledonia to evaluate how local conditions may sustain such large numbers of breeding individuals. We examined whether the population uses a bimodal foraging strategy to cope with presumably reduced near-colony food availability and/or intra-specific competition, and whether the foraging areas used by different colonies overlap (Bolton et al 2019). To examine this, we equipped WTS with miniaturized GPS during 2 different breeding seasons in 4 colonies.…”

The consequences of seabird habitat loss from offshore wind turbines, version 2 : Displacement and population level effects in 5 selected species

“…Theoretically it is possible that displacement due to OWFs leads to higher densities of birds around the OWF areas, and that this negatively affects the local food density. While there is evidence that food intake is affected by bird density for colony-breeding seabirds during the breeding (Wakefield et al 2013;Bolton et al 2018), there is no documented evidence of resource-competition outside the breeding season. Outside the breeding season birds have more space available (no nest to return to) and have more time to feed (no time lost flying back and forth; though the advantage is offset by shorter day length in winter).…”