Individual heterogeneity in fitness in a long‐lived herbivore

Heterogeneity in the intrinsic quality and nutritional condition of
individuals affects reproductive success and consequently fitness. Black
brant (Branta bernicla nigricans) are long-lived, migratory, specialist
herbivores. Long migratory pathways and short summer breeding seasons
constrain the time and energy available for reproduction, thus magnifying
life-history trade-offs. These constraints, combined with long lifespans
and trade-offs between current and future reproductive value, provide a
model system to examine the role of individual heterogeneity in driving
life-history strategies and individual heterogeneity in fitness. We used
hierarchical Bayesian models to examine reproductive trade-offs, modeling
the relationships between within-year measures of reproductive energy
allocation and among-year demographic rates of individual females breeding
on the Yukon-Kuskokwim Delta, Alaska using capture-recapture and
reproductive data from 1988 to 2014. We generally found that annual
survival tended to be buffered against variation in reproductive
investment, while breeding probability varied considerably over the range
of clutch size-laying date combinations. We provide evidence for
relationships between breeding probability and clutch size, breeding
probability and nest initiation date, and an interaction between clutch
size and initiation date. Average lifetime clutch size also had a weak
positive relationship with apparent survival probability. Our results
support the use of demographic buffering strategies for black brant. These
results also indirectly suggest associations among environmental
conditions during growth, fitness, and energy allocation, highlighting the
effects of early growth conditions on individual heterogeneity, and
subsequently, lifetime reproductive investment.