Changes in egg and body temperature indicate triggering of egg desertion at a body mass threshold in fasting incubating blue petrels (Halobaena caerulea)

The triggering of transitory egg desertion in fasting and incubating blue petrels (Halobaena caerulea, nocturnal burrowing seabirds living in the subantarctic region) was investigated by continuously monitoring both body temperature (T-sto) and egg temperature (T-egg) with a telemetry system, and by measuring body mass (BM) loss. The birds were kept captive in their burrow and incubated day and night without any interruption; there was no day-night cycle in T-sto and T-egg, which averaged 39.9 degrees C and 32.0 degrees C, respectively. There was no evidence of hypothermia as a way to save energy in this fasting situation. Egg desertion occurred at night and was an abrupt and definitive phenomenon reflected by a simultaneous fall in T-egg and a peak in T-sto. After egg desertion, a distinct day-night cycle of body temperature was observed, T-sto being 0.6 degrees C higher during night-time (P < 0.05), probably reflecting increased nocturnal activity. BM at egg desertion averaged 166.7 +/- 3.8 g in telemetered birds and 164.4 +/- 1.6 g in a group of free-living birds. Throughout fasting; the specific daily BM loss remained at 46 +/- 1 g . kg(-1) . day(-1), but increased sharply below a critical BM of 160.0 +/- 2.5 g. Thus, fasting incubating blue petrels spontaneously desert their egg when reaching a BM threshold. This BM is very close to a critical value in fasting birds and mammals that corresponds to a critical depletion of fat stores and to a shift from lipid to protein utilization. This strongly suggests that such a metabolic shift triggers behavioural changes leading to egg desertion and refeeding, which is of great relevance to the understanding of the long-term control of food intake and BM.