2.2 Encounter data
We used mark-recapture data from 899 adult penguins, 387 at Robben
Island and 512 at Stony Point (Table A1). Encounter data were collected
over the breeding season (March to October); survival rates consequently
refer to the nonbreeding season (November to February). We assigned each
penguin a state both during initial marking and each subsequent
encounter. Individuals were considered breeders if they were attending
chicks or eggs in a nest site or guarding a nest site during nest
inspections. Additionally, if they were encountered in the colony via
the ground reader ≥ 6 times over a minimum of 12 days and a maximum of
120 days but not confirmed as breeders during nest inspections, they
were considered breeders based on knowledge of African penguin breeding
biology (Williams & Cooper 1984). To ensure that any birds encountered
as nonbreeders were definitely skipping reproduction and were not
younger birds yet to breed for the first time, we included an additional
state of ‘prebreeder’, assigned to any birds marked/encountered as
nonbreeders that had not yet been encountered breeding within their
encounter histories.
Earlier tagging of African penguins used stainless steel flipper bands
(e.g., Sherley et al. 2014). However, these have now been phased
out due to potential data bias, both through human error (e.g.,
incorrectly reading tags) and potential deleterious effects on penguin
survival and behaviour (e.g., Dugger et al. 2006; Dann et
al. 2014). Within this dataset, four double marked (flipper band from
previous tagging, and transponder from this study) individuals were
removed to reduce any potential bias. Additionally, movement between
colonies is rare in breeding African penguins (Sherley et al.2014); here, one individual encountered breeding at >1
colony was removed from the dataset to improve the reliability of model
estimates by reducing the need for more complex models incorporating
inter-colony movement (as this was not the focus of our study).