Stable isotopes demonstrate intraspecific variation in habitat use and trophic level of non‐breeding albatrosses

Abstract

The non‐breeding period is critical for restoration of body condition and self‐maintenance in albatrosses, yet detailed information on diet and distribution during this stage of the annual cycle is lacking for many species. Here, we use stable isotope values of body feathers (δ13C, δ15N) to infer habitat use and trophic level of non‐breeding adult Grey‐headed Albatrosses Thalassarche chrysostoma (n = 194) from South Georgia. Specifically, we: (1) investigate intrinsic drivers (sex, age, previous breeding outcome) of variation in habitat use and trophic level; (2) quantify variation among feathers of the same birds; and (3) examine potential carry‐over effects of habitat use and trophic level during the non‐breeding period on subsequent breeding outcome. In agreement with previous tracking studies, δ13C values of individual feathers indicate that non‐breeding Grey‐headed Albatrosses from South Georgia foraged across a range of oceanic habitats, but mostly in subantarctic waters, between the Antarctic Polar Front and Subtropical Front. Sex differences were subtle but statistically significant, and overlap in the core isotopic niche areas was high (62%); however, males exhibited slightly lower δ13C and higher δ15N values than females, indicating that males forage at higher latitudes and at a higher trophic level. Neither age nor previous breeding outcome influenced stable isotope values, and we found no evidence of carry‐over effects of non‐breeding habitat use or trophic level on subsequent breeding outcome. Repeatability among feathers of the same individual was moderate in δ13C and low in δ15N. This cross‐sectional study demonstrates high variability in the foraging and migration strategies of this albatross population.

Mercury exposure in an endangered seabird: long-term changes and relationships with trophic ecology and breeding success

“…Rivers may deliver large amounts of Hg into the oceans [ 59 ]. Potentially, Hg in rivers flowing onto the Patagonian Shelf may be carried south in the Brazil Current and, at the confluence with the Falklands Current, be transported east in the South Atlantic Current to grey-headed albatross foraging areas [ 33 , 35 , 36 ]. Both hypotheses require further investigation, including via direct measurements of Hg in water samples or prey of grey-headed albatrosses.…”

“…Male grey-headed albatrosses at South Georgia are heavier, with larger wing areas and higher wing loadings than females [ 67 ]. During the non-breeding period, males forage at slightly higher trophic levels and at higher latitudes compared with females [ 36 ], and tracking data show that core areas but not overall distributions were segregated to some extent during the non-breeding summer only [ 35 , 37 ]. No effects of age were found in the present study, which is in agreement with results from other albatross species [ 26 – 28 , 39 ], and previous studies of grey-headed albatrosses with much smaller sample sizes [ 14 , 39 ].…”

“…Hence, selective mortality of particular phenotypes cannot be excluded except by conducting a longitudinal study [ 32 ]. Stable isotope ratios of adult seabird feathers provide information about foraging ecology in the non-breeding period [ 36 , 60 ]. However, owing to different integration periods, it has been suggested that relationships between stable isotopes and THg concentrations may be spurious [ 68 ]; nonetheless, in albatrosses, both stable isotope ratios and THg concentrations appear to reflect diet during feather synthesis [ 18 ; and references therein].…”

“…This species is extremely long-lived [32], and forages at mid-to high trophic levels-predominantly consuming cephalopods, but also fish and Antarctic krill (Euphausia superba) [33,34]. During the non-breeding period, free from the constraints of centralplace foraging, birds disperse across a wide range of oceanic habitats though mainly targeting the Subantarctic Zone (SAZ) between the APF and the Subtropical Front (STF) [35][36][37]. Hg exposure tends to be high in this species, with intrapopulation variation reflecting its generalist feeding habits and wide foraging range [18].…”

Can diet niche partitioning enhance sexual dimorphism?

“…An outstanding and unresolved question concerns the importance, Ecological divergence related to size dimorphism could be exhibited by sex differences in ecological mean if, for example, dimorphism impacts the prey available to each sex, leading them to feed, on average, at different trophic levels (Mills et al, 2021). Furthermore, sexual size dimorphism could lead to sex differences in ecological variation via numerous mechanisms.…”