Energetic Cost of Incubation in the Zebra Finch

-At temperatures below 28”C, rate of oxygen consumption (vjo2) of Zebra Finches (Poephila guttuta) incubating eggs averaged 20% higher than the \ioz of non-incubating Zebra Finches sitting in a nest at the same temperature. This increase represents the energetic cost of incubation. The O,, of non-incubating birds sitting in a nest was lower than values reported for birds perched in the open at the same temperature. In the Zebra Finch, the ameliorating effects of the nest microclimate approximately compensate for the increment in metabolic rate due to incubation. The energetic cost of incubation increased when birds had to rewarm cold eggs. Incubating birds responded to artificially cooled eggs by elevating their metabolic rate and increasing heat flow to the clutch. The pattern of adult attentiveness at the nest determines the number of times and amount by which the eggs must be rewarmed. Because it is energetically more expensive to rewarm eggs than to maintain temperature once the eggs are warm, the cost of incubation depends in part on the attentiveness pattern. Reproduction, especially the care of eggs and young, places special demands on the way birds allocate available energy. Various techniques have been used to estimate the energetic costs associated with territorial defense (Stiles 1971, Wolf and Hainsworth I971), nest building (Collias and Collias 1967, Withers 1977a), egg production (King 1973), and the feeding of nestlings (Utter and Lefebvre 1973). These activities, together with incubation of the eggs, include most of the time and energy that birds devote to reproductive activities. However, the costs of incubation, an activity that takes up a significant portion of the reproductive cycle, are difficult to measure and have been the subject of controversy (Kendeigh 1973, King 1973). Linear heat flow models have been used to make indirect estimates of the energetic cost of incubation based on the assumption that heat loss from eggs must be balanced by extra heat production by the parent (Kendeigh 1963, Ricklefs 1974). For example, Drent (1970) found that Kendeigh’ s (1963) model accurately described the heat input required to keep the eggs of Herring Gulls (Larus urgentu Mertens 1977) and Baird’ s Sandpiper, (Calidris hairi Norton 1973). However, neither author compared the oxygen consumption of incubating and non-incubating birds, and consequently it is impossible to estimate the energy expended solely for incubation. In contrast, Biebach (1977, 1979) measured the oxygen consumption of both incubating and non-incubating Starlings (Sturnus dgaris) and calculated the energetic cost of incubation at several nest temperatures. Gessaman and Findell (1979) measured CO, production of three incubating and non-incubating American Kestrels (F&o sparzjerius) but their results are ambiguous. Mertens (1980) measured heat loss from the nest of a Great Tit, using heat flux disks mounted in the nestbox walls, and found that heat loss from the bird and nest increased considerably during incubation. The energetic cost of warming cooled eggs “to the incubation temperature” has not been determined. Kendeigh et al. (1977) suggested that energy expended to rewarm eggs could be calculated from estimates of the heat needed to rewarm eggs, but this procedure ignores the inefficiency of heat transfer from the parent to the eggs. If a bird increases its heat production in order to re-