Early-growth trajectories affect juvenile survival, age at first reproduction and lifetime fitness in a long-lived seabird, the little penguin: dataset.

This dataset was used to conduct the study "Early-growth trajectories affect juvenile survival, age at first reproduction and lifetime fitness in a long-lived seabird, the little penguin" published in Journal of Animal Ecology in 2025.The first file, “growth_curves.xlsx” corresponds to the mass measurements of the chicks (approximately one measurement every 2 days during the post-guard period), with one line per mass measurement. Each measurement is accompanied by information about the nest (nest), the breeding season (season), the date of measurement (date), the order in which the chick hatched (A or B), the weight (wgt), the age of the chick (age), the chick's tag if it has one (tag), the clutch number (1, 2, or 3), the hatching date (HD), and the tags of both parents (father and mother).The second file, “weighbridge.xlsx”, corresponds to individuals passing between the two antennas located at the Phillip Island colony using their pit tags. Each line corresponds to the passage of an individual. It includes the date of passage (date_utc) and the individual's tag (pit_tag). The third file, "pairs.xlsx" corresponds to the monitoring of the breeding season. Each line corresponds to a breeding event. It includes information about the nest, the tags of the two breeding adults (male, female), the season (season), the laying date (laying_date), the hatching date (hatching_date), the end of the guarding period (end.guard), the number of eggs laid (eggs), the number of chicks hatched (nb_chicks), the number of chicks that fledged (fledged), the clutch number (clutch), and the fledging dates of the two chicks A and B (FD_A, FD_B).The aim of this study was to study the natural variation of growth patterns displayed within a seabird population and assess their impact on juvenile survival, age at first reproduction, lifetime reproductive outputs (LRO) and longevity. We compiled over 2200 chick growth curves over the last 26 years and defined 11 growth parameters. A non-supervied statistical clustering showed that growth trajectories clustered into three main groups: fast, slow and light. Fast chicks (n=48%) attained the highest maximum mass in the shortest amount of time, whereas slow chicks (n=33%) stood out by a prolonged (+ 7days, i.e. +13% in comparison to fast chicks) and irregular period of juvenile growth. Finally, light chicks (n=19%) reached low maximum and fledging masses (~ -350g; -37% and -36% of fast and slow chicks). Then, we tested for the effects of chick growth parameters on subsequent annual vital rates estimated through Capture-Mark-Recapture methods as well as longer-term effects on life-history outcomes using Markov chain models. Fast and slow individuals had the highest survival rates from hatching to yearling age (19% and 17% respectively), while light chicks were at a disadvantage during this initial period (3% survival). Fast individuals reproduced 12% earlier (2.6 yo) than slow individuals, had 12.5% to 88% greater longevity (up to 21 yo), and produced 1.2 to 3.8 times more eggs over their lifespan than slow and light individuals, respectively. Fast chicks reached maturity faster and produced more offspring during their lifetime without discernable negative effects on their longevity, highlighting possible silver-spoon effects.