Variable energetic responses to clutch size manipulations in white-throated dippersCinclus cinclus

Abstract

We used the doubly-labelled water technique to measure daily energy expenditure (DEE) of a free-living uniparental incubator, the white-throated dipper Cinclus cinclus, in Scotland. DEE was 205 ± 8 (s.e.m.) kJ d -1 for 17 females incubating their natural clutch sizes, equivalent to 3.2 ± 0.1 x basal metabolic rate (BMR). To investigate the influence of clutch size on the energy budget, we measured the DEE of 14 females with clutches increased or reduced by a single egg. Birds with reduced clutch sizes had an energy expenditure with a mean and variance that did not differ from those of birds with unmanipulated clutches. Enlarging the clutch led to an increase in energy expenditure to over 4 x BMR for some individuals but not for others, resulting in greater variance in energy expenditure for birds with enlarged clutches. Individual variation in energy expenditure could not be fully explained by environmental conditions, by patterns of behaviour or clutch size. Incubating females received a maximum of only 4 kJ d-' (2% of DEE) from provisioning by the male, and mobilised up to 6 kJ d -1 (3% of DEE) from reserves. Females spent 2.9 ± 0.2 h (n = 20) away from the nest each day, so a foraging rate of 95 kJ h -1 was required during incubation recesses to balance DEE. This 'required foraging rate' is double previous estimates of the maximum rates of energy acquisition for birds of this size. We suggest that the greater likelihood of a raised energy expenditure associated with larger clutches, combined with the difficulties in maintaining energy supplies, may constitute a constraint on avian clutch size.