Summary 1.To investigate reproductive costs associated specifically with incubation for a singlesex intermittent incubator, clutches of the Barn Swallow ( Hirundo rustica Linnaeus) were manipulated in successive trios, involving reduced ( n − 2), control ( n ) and enlarged ( n + 2) clutch sizes ( n is the natural clutch). All manipulated clutches were within the natural range. 2. Manipulations were made shortly after clutch completion and reversed immediately prior to hatching, so the costs of incubation were examined in isolation while the demands of egg-laying and nestling rearing were maintained at natural levels. 3. The efficiency of incubation was dependent on clutch size: the period from laying to hatch was shortest for those incubating reduced ( R ), and longest for birds incubating enlarged ( E ) clutches ( R : 14·8 ± 0·1 days, n = 28; control ( C ): 15·3 ± 0·3, n = 24; E : 15·6 ± 0·2 days, n = 28). Also, hatching success was higher amongst R (92%) than either C (85%) or E (81%) clutches. 4. There was no evidence of an incubation cost due to clutch size in terms of parental body condition, provisioning of nestlings, or in the time interval before a second clutch. 5. No difference was found in female nest attendance behaviour or in the decline in egg density (which could reflect water loss) according to our clutch manipulations, indicating that other factors probably underlie the effects observed. 6. It was shown that the number of eggs being incubated affects components of fitness in Barn Swallows, independently of laying and chick-rearing costs. Intraseasonal costs of incubation included prolonged egg development and reduced hatching success among larger clutches but did not affect nestling growth or interclutch intervals. We suggest that an inability to maintain optimal temperature for egg development is likely to underlie the observed responses to clutch size manipulations.

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×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×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−1 (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.