Is the reduced incubation time for B-eggs in Rockhopper Penguins Eudyptes chrysocome linked to egg density variation?

Crested penguins (genus Eudyptes) present a unique reversed egg-size dimorphism and hatching asynchrony, with the larger second-laid egg (B-egg) hatching before the smaller first-laid egg (A-egg). Both a higher water vapour conductance and parental favouritism during incubation could explain the shorter incubation period for the B-egg than for the A-egg. Because the incubation period is increased by the presence of a sibling for A-eggs, but not for B-eggs, and because both egg categories have the same incubation period when they are incubated alone, it has been suggested that the difference in incubation period was largely driven by the parental favouritism for B-eggs during incubation. We tested whether A- and B-eggs show a difference in laying density, density at the beginning of incubation and in density decrease during incubation according to the presence of a sibling. Although density at the start of incubation was significantly higher for B-eggs than for A-eggs, the decrease in density during incubation had the same slope for both egg categories. Moreover, the presence of a sibling did not influence densities. We additionally provide two equations that allow the back-dating of laying dates for a clutch of SouthernRockhopper Penguin (Eudyptes c. chrysocome) and we discuss the precision of the method (3.04 +/- A 2.29 days for A-eggs and 2.73 +/- A 2.10 days for B-eggs) for penguins which are increasingly being used as marine environmental sentinels.