THE USE OF A TIME AND ENERGY BUDGET MODEL OF A PARENT BIRD TO INVESTIGATE LIMITS TO FLEDGING MASS IN THE THICK-BILLED MURRE

1. We develop a general model to show bow the energy delivered by a pa rent bird to its young may be limited by either time constraints or en ergy constraints acting on the parent. The model gives explicit equati ons for each constraint.2. We illustrate the model using data from the Thick-billed Murre, which raises a single chick. The chick leaves til e nest site at. less than one third of the adult mass and unable to fl y. Although this behaviour has been discussed in terms of adaptations to foraging constraints that operate on the breeding adults, these con straints have hitherto not been examined in detail. 3. We show that th e maximum size of chick depends strongly on the distance that a parent must travel an a foraging trip. When the round-trip distance: is more than 100 km, the Thick-billed Murre is unlikely to be able to raise t he chick's fledging mass above 65% of the adult mass, and hence is una ble to adopt a semi-praecocial chick-rearing strategy, 4. Our results suggest that if the parent bird is limited by energy expenditure then the time spent brooding does not have much of an effect on maximum chi ck size.