DOI: 10.5061/dryad.m37pvmd2h
title: Age-related breeding success in little penguins: A result of selection and ontogenetic changes in foraging and phenology

description: 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.
description: This data comes from a long-term monitoring of little penguin
 breeding at Summerland Peninsula on the western tip of Phillip Island (33°
 31'S, 145° 09'E), Victoria, Australia. This study covers 19
 breeding seasons from 2000 to 2018 (inclusive), where each breading season
 spans from August to March the year after (the austral summer). Sex was
 determined by bill measurements when the birds were first found in the
 colony as adults, while age was known from bird tagging as chicks. All
 penguins included in this study nested in 100 artificial burrows (nest
 boxes) established behind the Penguin Parade® area. The artificial nest
 boxes were monitored three times a week during the breeding season to
 assess breeding phenology and success of each pair. Individuals with
 transponders were detected in the nest using a purpose-built handheld
 transponder reader (Kean Electronics, Australia). This monitoring ensured
 a recapture probability close to one and informed us about laying date,
 the number of clutches per season (1 to 3), as well as the number of
 fledglings per breeding event (0 to 2). Chicks were considered fledged
 when they had all their adult feathers and were older than 40 days when
 last seen. Fledging success was estimated as the number of chicks that
 survived to fledging per breeding event. Attendance data were recorded by
 an Automatic Penguin Monitoring System (APMS) located along the main path
 into the Penguin Parade study site, and were used to calculate three
 foraging parameters: foraging trip duration, change in adult body mass,
 and chick meal size. The APMS consists of an electronic transponder reader
 and a weighing platform that records the transponder identification, mass,
 and time of a penguin crossing the weighbridge. Foraging trip duration was
 calculated as the number of days at sea between colony visits. Foraging
 duration was measured in days, as little penguins depart before sunrise
 and return after sunset. Short trips were defined as those lasting less
 than or equal to 2 days (&lt;2 days) whereas long
 trips lasted more than 2 days. Two mass parameters were determined for
 each trip: parental body mass change and chick meal size. Adult body mass
 change was the amount of mass change per foraging trip and was calculated
 as the difference in body mass between when a penguin goes out and when it
 returns from the foraging trip. Chick meal size was the amount of food
 given to the chick(s) and was assumed to be the mass difference between
 the penguin arriving at the colony after the foraging trip and returning
 once again to sea. Chick meal size could solely be estimated during
 post-guard, when little penguins only spend a few hours in the colony, the
 mass loss while in the colony assumed to be due to feeding their chicks
 rather than their metabolism.
description: Note that the Automatic Penguin Monitoring System records started
 in June 2001, and foraging variables are thus absent in the 2000
 season. APMS is calibrated weekly to account for tare drifts, and deviance
 is considered for weight assessment. However, automatic weighing can
 sometimes provide inconsistent data due to multiple birds crossing at
 once. Only individual masses ranging between 700 and 1700g were
 considered, based on field observation of body mass range. Only trips that
 lasted between 1 and 16 days were considered for incubation and
 post-guard, and trips up to 3 days in guard (longer trips were classed as
 false negatives or abandonment and end of breeding). Based on the manual
 weighing of individuals in the colony, changes in body mass outside of the
 0 to 600g range during chick-rearing (guard and post guard) and -75 to
 500g range during incubation were considered inconsistent and
 discarded. Only chick meal sizes smaller than 550g were included in data
 analysis following previous direct observations.