Use of amino acid isotope analysis to investigate capital versus income breeding strategies in migratory avian species

Income and capital breeding represent opposing ends of a continuum of
reproductive strategies. Quantifying nutrient allocation to reproduction
is challenging, but recent advances in compound-specific stable isotope
analysis hold promise for tracing the source of individual compounds
allocated to reproduction.   Here, we describe a novel approach
of using measured carbon (δ13C) and nitrogen (δ15N) isotope values of
individual amino acids (AAs) in pectoral muscle of egg-laying females and
egg yolk as a useful tool to quantify the reliance on income versus
capital breeding in migrating species. We used white-fronted (Anser
albifrons frontalis), lesser snow (A. caerulescens caerulescens), and
black brant (Branta bernicla nigricans) geese breeding in tundra
ecosystems of northern Alaska as model organisms. All three species relied
on mixed capital-income breeding strategies, but models based on AA
isotope data estimated higher proportional contributions of endogenous
resources to yolk synthesis compared to results based on bulk tissue
isotope analyses.  Tracing income versus capital nutrient
allocation in migratory species at the compound level is a major advance
from the current ‘elemental’ perspective obtained from bulk tissue stable
isotope analyses. Our framework is applicable to all taxonomic groups, as
long as there is a sufficient spatial or temporal isotopic gradient
between resources obtained during the breeding and non-breeding periods.