Geoff M. Hilton

USING ARTIFICIAL NESTS TO TEST IMPORTANCE OF NESTING MATERIAL AND NEST SHELTER FOR INCUBATION ENERGETICS

Abstract Incubation is an energetically demanding process for parents, in part because of the thermodynamic costs of maintaining egg temperature. One might predict that aspects of nest construction—in particular, the thermodynamic properties of the nesting material and the degree to which the nest provides shelter from the wind—would have important effects on thermodynamic costs. However, little is known about the relative importance of those factors. Here, we investigate egg cooling rates in several commonly used nesting materials and in various wind speeds and examine the effect on those rates of wetting the materials. Nesting materials differ greatly in their insulating properties; feather down is the best insulator, and grass the worst. When the materials are wet, eggs cool much more rapidly, differences between materials tend to diminish, and down becomes the worst insulator. Hence, there may be significant selection pressure to choose particular nesting materials, but materials may be better or worse according to the situation of the nest. Increasing wind speed also has profound effects on egg cooling rates, even at the low speeds tested here, which implies strong selection pressure to locate and construct nests that minimize wind speed at the egg surface. Our results suggest that nest construction may have an important bearing on the subsequent costs of reproduction, and that important trade-offs may exist between nest construction for reduced thermodynamic costs, and other costs and benefits of nest-building and reproduction.

ENERGETIC CONSEQUENCES OF CLUTCH TEMPERATURE AND CLUTCH SIZE FOR A UNIPARENTAL INTERMITTENT INCUBATOR: THE STARLING

Abstract In uniparental intermittent incubators, incubating parents must simultaneously regulate both the temperature of the clutch and their own energy level. To examine energetic consequences of providing different thermal environments for clutches of different sizes, a dynamic model was constructed in which energy level of an incubating European Starling (Sturnus vulgaris) and temperature of its clutch were simultaneously described. Adult energy balance after a day of incubation decreased as mean clutch temperature increased, such that the debt accrued while maintaining a clutch within the optimal developmental temperature range (36–39°C) was predicted to be prohibitively high, despite the prediction that the mean basal metabolic rate (BMR) required to maintain this temperature was <2 BMR. Thus, this model can explain our observation that starlings maintained their eggs at 32–33°C, well below the developmental optimum. Consistent with empirical studies, our model predicted that the metabolic demand of incubation increases with clutch size. That increase was predicted to affect adult energy balance and hence cost of incubation when starlings maintained their clutches at optimal temperatures, but not at the lower temperatures actually observed. Hence, clutch-size-dependent variation in incubation demands may be unlikely to influence optimal number of eggs that a starling should lay. However, exact relationship between clutch size and adult energy debt depended on the nature of the relationship between clutch size and clutch thermal properties and on mean incubation temperature. Thus, consequences of clutch size for the cost of incubation are not clearly predictable, and caution may be required when using experimental clutch enlargements to manipulate reproductive costs.

Ecological constraints on digestive physiology in carnivorous and piscivorous birds

Digestion strategies of meat- and fish-eating birds have received little attention, and the assumption has generally been made that there is rather little variation in digestion parameters between species in these guilds. We show that there is significant though small variation between species in apparent absorption efficiency. This variation is associated with an apparent trade-off between retention time of digesta and apparent absorption efficiency: short retention times result in low apparent absorption efficiency. We show that, in raptors, rapid digestion is a consequence of both reduced gut length and increased flow rate of digesta. We examine the ecological correlates of digestive strategy in raptors and seabirds. Rapid digestion appears to be associated with a pursuit foraging mode, whereas slow digestion tends to occur in species with a searching foraging mode. We suggest that in raptors which actively pursue aerial prey, the weight savings that can be achieved through rapid digestion exceed the costs in reduced apparent absorption efficiency. However, a species which adopts a strategy of rapid but inefficient digestion may be restricted in diet to high-quality food types, whereas species with a slow but efficient digestive strategy are able to exploit a wider range of food types, including low-quality prey. J. Exp. Zool. 283:365–376, 1999.