Age-related breeding success in little penguins: A result of selection and ontogenetic changes in foraging and phenology

Reproductive performance typically improves with age, reaching a plateau
at middle age and subsequently declining in older age classes (senescing
individuals). Three potential non-exclusive mechanisms can explain the
improvement in reproductive performance with age: (1) selection (poor
quality individuals are removed from the population with increasing age),
(2) constraint (individual efficiency increases through experience) and
(3) restraint (reproductive investment increases with age as the residual
reproductive value decreases). While all three mechanisms received strong
empirical support, few studies have aimed at teasing apart those
hypotheses and understanding their underlying functioning. In little
penguins (Eudyptula minor), we used a 19-year longitudinal dataset on
breeding and foraging of more than 450 individuals to investigate the
effect of age on breeding success. We separated within- from
among-individual age-effects using state-of-the-art statistical methods
(within-subject centering and population change decomposition). We then
assessed whether within-individual changes in breeding resulted from
ontogenetic changes in foraging performances, breeding phenology or access
to mates and nest sites. Fidelity and assortative pairing explained the
high correlation in male and female ages within a pair. Breeding
performances followed a typical bell-shaped curve with performance
increasing up to 8 years-old, before reaching a plateau and subsequently
declining after age 16. Both selection and within-individual processes
occurred, although within-individual changes dominated differences in
age-dependent breeding success. The selective appearance had almost no
effect (apart from ages 2 to 3), and selective disappearance mostly
affected changes at old ages (above 16), although they were also
responsible for the slight increase in reproductive performances from ages
5 to 8. Focusing on within-individual changes, birds exhibited higher
performances at middle ages, with birds foraging better, laying earlier
and changing partner and nest less often. Their reproductive investment
did not vary with age for females and slightly decreased for males. This
supports the constraint hypothesis but not the restraint one. Finally, the
increase in breeding performances at young ages was explained by the
age-related increase in foraging performances during chick-rearing and
advancement of laying. In contrast, reproductive senescence was defined by
a general decrease in bird performances.