Tracking, feather moult and stable isotopes reveal foraging behaviour of a critically endangered seabird during the non‐breeding season

Abstract

Aim The movement patterns of marine top predators are likely to reflect responses to prey distributions, which themselves can be influenced by factors such as climate and fisheries. The critically endangered Balearic shearwater Puffinus mauretanicus has shown a recent northwards shift in non-breeding distribution, tentatively linked to changing forage fish distribution and/or fisheries activity. Here, we provide the first information on the foraging ecology of this species during the non-breeding period.Location Breeding grounds in Mallorca, Spain, and non-breeding areas in the north-east Atlantic and western Mediterranean.Methods Birdborne geolocation was used to identify non-breeding grounds. Information on feather moult (from digital images) and stable isotopes (of both primary wing feathers and potential prey items) was combined to infer foraging behaviour during the non-breeding season.Results Almost all breeding shearwaters (n = 32) migrated to non-breeding areas in the Atlantic from southern Iberia to the French Atlantic coast, where the majority of primary feather moult took place. Birds foraging off western Iberia yielded feather isotope ratios consistent with a diet composed largely of pelagic fishes, while the isotopic composition of birds foraging in the Bay of Biscay suggested an additional contribution of benthic prey, most likely from demersal fishery discards.Main conclusions Combined application of geolocators and stable isotopes indicates spatial variation in dietary behaviour and interactions with fisheries. Our results imply that both pelagic fish and fisheries discards are important components of diet during the non-breeding period, which may have implications for the at-sea distribution of this migratory species. These findings will contribute to bycatch mitigation in non-breeding areas and provide baseline data that should inform future assessment of seabird responses to changing fishery practices and prey distributions.

Short‐term prey field lability constrains individual specialisation in resource selection and foraging site fidelity in a marine predator

“…We used a diet‐tissue discrimination factor of δ 15 N: 3.7 ± 1‰ (mean ± SD ) as recommended by Meier et al . () for shearwaters. The mean stable isotope ratios with their standard deviations assessed independently for prey are also shown: zooplankton, copepods, euphausiids (empty squares), small pelagic fish (empty triangles) and squids (empty circles).…”

“…We show the mean expected range (solid lines) and its standard deviation (dotted lines) in prey isotopic values for shearwaters, based on shearwater tissue. We used a diet-tissue discrimination factor of d 15 N: 3.7 AE 1& (mean AE SD) as recommended by Meier et al (2017) for shearwaters. The mean stable isotope ratios with their standard deviations assessed independently for prey are also shown: zooplankton, copepods, euphausiids (empty squares), small pelagic fish (empty triangles) and squids (empty circles).…”

Costs of breeding are rapidly buffered and do not affect migratory behavior in a long‐lived bird species

“…The purpose of the sampling of five primaries along the wing sequence was to assess individual molting patterns in relation to migratory movements, based on the differences in the isotopic ratios of the prey across water masses (Meier et al. ). We sampled on the right and left wings and removed only a small portion of each remige to minimize potential detrimental effects of the sampling on wing load.…”

“…Additionally, at the time of geolocator recovery, we sampled two entire feathers from every bird (the first primary feather, P1, and the thirteenth secondary feather, S13) for CORT determinations, and small basal portions (~10 mm) of the following primaries: P1, P5, P9 from the right wing, and P3, P7 from the left one for stable isotope analysis (SIA). The purpose of the sampling of five primaries along the wing sequence was to assess individual molting patterns in relation to migratory movements, based on the differences in the isotopic ratios of the prey across water masses (Meier et al 2017). We sampled on the right and left wings and removed only a small portion of each remige to minimize potential detrimental effects of the sampling on wing load.…”

Causes and consequences of individual variability and specialization in foraging and migration strategies of seabirds

“…Both conventional diet (stomach content analysis) and stable isotope studies indicate that wintering seabirds can change their diet or widen their trophic niche, since individuals are no longer central-place foragers and are free to select their favoured habitat or prey (Cherel et al 2007, Karnovsky et al 2008, Hedd et al 2010, Harris et al 2015. It is important to note that we lack knowledge for most seabirds of their prey during the winter; although stable isotope studies offer a partial solution, ideally these need to control for changes in isotopic baselines because of the scale of seabird movements (Meier et al 2017).…”