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 b lue petrels (Halobaena caerulea, nocturnal burrowing seabirds living i n the subantarctic region) was investigated by continuously monitoring both body temperature (T-sto) and egg temperature (T-egg) with a tele metry system, and by measuring body mass (BM) loss. The birds were kep t captive in their burrow and incubated day and night without any inte rruption; there was no day-night cycle in T-sto and T-egg, which avera ged 39.9 degrees C and 32.0 degrees C, respectively. There was no evid ence of hypothermia as a way to save energy in this fasting situation. Egg desertion occurred at night and was an abrupt and definitive phen omenon 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 wa s observed, T-sto being 0.6 degrees C higher during night-time (P < 0. 05), probably reflecting increased nocturnal activity. BM at egg deser tion 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 dai ly BM loss remained at 46 +/- 1 g . kg(-1) . day(-1), but increased sh arply below a critical BM of 160.0 +/- 2.5 g. Thus, fasting incubating blue petrels spontaneously desert their egg when reaching a BM thresh old. This BM is very close to a critical value in fasting birds and ma mmals that corresponds to a critical depletion of fat stores and to a shift from lipid to protein utilization. This strongly suggests that s uch a metabolic shift triggers behavioural changes leading to egg dese rtion and refeeding, which is of great relevance to the understanding of the long-term control of food intake and BM.