Parental responses to unexpectedly cool eggs in Nazca boobies Sula granti

Although incubation temperatures have been documented extensively in birds, few studies have followed fluctuations in temperatures throughout the length of the incubation period in natural nests. We recorded incubation temperatures of Nazca boobies Sula granti by replacing real booby eggs with a model egg containing an internal floating data logger for three‐day intervals in 47 nests (“experimental group”). We also added the same logger eggs to 14 booby nests at the time of egg‐laying, where they remained as the second egg in the clutches for the entire incubation period (“logger egg control group”). Finally, we measured surface temperatures of real eggs with an infrared sensor (“real egg control group”). In both control groups, the average temperature increased after laying, then stabilized for the remainder of the incubation period. The experimental group differed from the controls, because the cool logger egg could have been introduced at any point in the incubation cycle, not just at the beginning. Egg temperature in the experimental group had a parabolic relationship with day of incubation, because parents receiving a logger egg during the third quarter of incubation showed an exaggerated heating response during the subsequent two days. We infer from this that the parents are especially sensitive to egg temperature during this period, and it may thus represent a critical period of unknown nature for the embryo.