Gull diets reveal dietary partitioning, influences of isotopic signatures on body condition, and ecosystem changes at a remote colony

Abstract

As top predators that feed on a wide range of prey items, gull diets may serve as important biological indicators of regional prey availability and changes in marine ecosystems. We studied the diets of herring gulls Larus argentatus and great black-backed gulls L. marinus on Sable Island, Nova Scotia, Canada, a remote colony which has shown high levels of contaminants in herring gull eggs and which has experienced significant ecological and anthropogenic change in its surrounding marine region over the past 40 yr. Analysis of regurgitated pellets suggested that current gull diets have proportionally less offshore prey (e.g. fish) and terns and tern eggs, and proportionally more molluscs, rock crabs Cancer borealis, and seal Halichoerus grypus carrion than diets sampled 40 yr ago. The composition of recent diets observed from pellet analysis is supported by stable isotope mixing models of carbon (δ 13 C) and nitrogen (δ 15 N), which revealed that great black-backed gulls had high proportions of seals and crab in their diets, whereas herring gulls had high proportions of crab, sand lance Ammodytes sp., and terrestrial invertebrates. Isotopic analyses also identified dietary variability through seasonal, age-specific and body condition relationships for each species. Biometric−isotope relationships showed that larger great black-backed gulls fed at higher trophic levels, and that higher trophic level foraging in herring gulls was associated with better body condition. Collectively, these results indicate dietary partitioning within this community of sympatrically nesting gulls, and broad-scale dietary shifts since the early 1970s.

Dietary Niche Shifts of Multiple Marine Predators under Varying Prey Availability on the Northeast Newfoundland Coast

“…As predicted, the niche breadth of gull chicks decreased more relative to other predator groups after capelin arrived in the study area, but remained broader than shearwaters and whales, suggesting that diet diversity remained relatively high. This supports previous findings that gulls are dietary generalists at the populationlevel (Fox et al, 1990;Pierotti and Annett, 1990;Steenweg et al, 2011;Ronconi et al, 2014). In our study, gull chicks also represented the only predator group whereby foraging (by parents) was constrained within range of the breeding colony (Orians and Pearson, 1979).…”

Peer Review #1 of "Recently-adopted foraging strategies constrain early chick development in a coastal breeding gull (v0.1)"

“…Higher trophic level diets, reflected by higher  15 N values (Ambrose and DeNiro 1986), have previously been linked to improved chick condition and higher breeding success in seabirds (e.g., Bukacinska et al 1996;Janssen et al 2011;Ronconi et al 2014;Van Donk et al 2017). Here, we did not find an intrinsic difference between marine and terrestrial diets when provided ad libitum.…”

“…Instead, dietary effects on chick development only became apparent under field conditions, and may thus relate to differences in cost-benefit ratios between marine and terrestrial foraging strategies. Food resource partitioning among individuals within a colony has often been ascribed to competitive differences in relation to body size, albeit mainly driven by differences between males and females in sexually dimorphic species Monaghan et al 1985;Ronconi et al 2014). Foraging at sea is known to be highly competitive, with frequent agonistic interactions that favor the largest birds (Garthe & Hüppop 1998;Hudson & Furness 1988) and, in the case of the Lesser Black-backed Gull, generally males .…”

Feather‐based measures of stable isotopes and corticosterone reveal a relationship between trophic position and physiology in a pelagic seabird over a 153‐year period

“…), δ 15 N is related to body condition (Ronconi et al . , ), and both δ 15 N and CORT f have been shown separately to be positively related to subsequent reproductive performance (Crossin et al . , Kouwenberg et al .…”