High juvenile annual survival probabilities in Southern Rockhopper Penguins Eudyptes chrysocome are independent of individual fledging traits

Juvenile survival is an important demographic parameter. Southern Rockhopper Penguins Eudyptes chrysocome have undergone a dramatic population decline in the past century across their distribution, but the demographic processes are poorly understood. To estimate juvenile annual survival probabilities, Rockhopper Penguin chicks from two cohorts on New Island, Falkland Islands, were marked with transponders and recorded in subsequent years using an automated gateway. We first estimated annual survival and detection probabilities using a Cormack-Jolly-Seber (CJS) model, and found that both probabilities were extremely high (81% in the first and 98% in the second, third and fourth years of life), even in comparison with adult birds. Because detection probability after 3 years was effectively 1, and our sample size (n = 114) was too small to explore the effects of individual traits on survival in a CJS model, we assessed whether sex, cohort, body mass and laying sequence affected whether juveniles returned to the colony during their first 3 years of life using a simple generalized linear model that assumed perfect detection. Juveniles from the first cohort and males showed a higher return probability than juveniles from the second cohort and females. There was no clear effect of fledging body mass on return rate, probably related to the favourable environmental conditions during the study period. The laying sequence did not markedly affect the return probability of chicks, indicating that, once fledged, first-laid A-chicks have the same probability to return as second-laid B-chicks despite a much larger initial maternal investment in B-eggs in this species. This study demonstrates extraordinarily high juvenile survival probabilities and will help to understand the recent changes in the population dynamics of the Falkland Islands Southern Rockhopper Penguins.