DOI: 10.5061/dryad.0rxwdbs3z
title: Intercolony variation in reproductive skipping in the African penguin mark-recapture data

description: In long-lived species, reproductive skipping is a common strategy whereby
 sexually mature animals skip a breeding season, potentially reducing
 population growth. This may be an adaptive decision to protect survival or
 a non-adaptive decision driven by individual-specific constraints.
 Understanding the presence and drivers of reproductive skipping behaviour
 can be important for effective population management, yet in many species
 such as the endangered African penguin (Spheniscus demersus), these
 factors remain unknown. This study uses multistate mark-recapture methods
 to estimate African penguin survival and breeding probabilities at two
 colonies between 2013 and 2020. Overall, survival (mean ± SE) was higher
 at Stony Point (0.82 ± 0.01) than Robben Island (0.77 ± 0.02).
 Inter-colony differences were linked to food availability; under
 decreasing sardine (Sardinops sagax) abundance, survival decreased at
 Robben Island and increased at Stony Point. Additionally, reproductive
 skipping was evident across both colonies; at Robben Island, the
 probability of a breeder becoming a nonbreeder was ~0.22, versus ~0.1 at
 Stony Point. Penguins skipping reproduction had a lower probability of
 future breeding than breeding individuals; this lack of adaptive benefit
 suggests reproductive skipping is driven by individual-specific
 constraints. Lower survival and breeding propensity at Robben Island
 places this colony in greater need of conservation action. However,
 further research on the drivers of inter-colony differences is needed.
description: Mark-recapture data of African penguins were collected over an
 8-year period (2013-2020) across two colonies in the Western Cape, South
 Africa. From 2013, penguins were captured in each colony and injected with
 passive integrated transponders (PITs). As part of routine nest
 monitoring, the presence and breeding status (breeding or nonbreeding) of
 tagged birds was identified, and captured untagged birds were
 tagged. Data were analysed using multi-state mark
 recapture models, with a group effect to evaluate colony differences, and
 3 breeding states (breeder, nonbreeder, and pre-breeder). Pre-breeder was
 a state assigned to any non-breeding individual not yet encountered
 breeding to ensure all 'non-breeders' were skipping reproduction
 and not young birds yet to breed for the first time. 
 Known parameters were fixed to improve model performance; since
 only breeders were marked in 2013 across both colonies (with nonbreeders
 marked in subsequent years, Table A1), survival and transition rates for
 nonbreeders and prebreeders were fixed to zero during 2013–14, as was
 recapture in 2014 in both colonies. Additionally, no nonbreeders were
 marked in 2014 at Robben Island, so prebreeder survival and transition
 during 2014–15, and prebreeder recapture in 2015 were also fixed to zero
 for this colony. Finally, impossible transitions
 (<em>ψ</em>[prebreeder to nonbreeder],
 <em>ψ</em>[nonbreeder to prebreeder], and
 <em>ψ</em>[breeder to prebreeder]) were fixed to zero at both
 colonies.
description: Program MARK and the ‘RMark’ package in R (White &
 Burnham 1999; Laake 2013) were used to analyse the data.