Rebstock and Boersma (2011) recently explored variation in the temperature and incubation period of eggs within clutches of the Magellanic Penguin (Spheniscus magellanicus). They defined incubation period as “the time from the laying of an egg to its hatching” and concluded that parental behavior explained why the incubation period of second eggs was shorter than that of first eggs and controlled asynchronous hatching. While we agree that parents influenced asynchronous hatching by delaying the onset of incubation, we argue that their conclusions are based on an unconventional definition of incubation period. They included the period before the delayed onset of incubation in the incubation period, which leads to confusion. We state that the incubation period cannot include the time before the (delayed) onset of incubation when parents are not warming the eggs. As regards this latter and widely accepted definition, Rebstock and Boersma (2011) provided a large dataset showing that incubation consistently lasted for 39 days, whenever the onset of incubation, for both first and second eggs. The divergence in the definition of “incubation period” and the failure to consider the “onset of incubation” for a species in which the first egg is not incubated immediately after laying led to confusion in the interpretation of the results and conclusions from Rebstock and Boersma (2011).