A trade‐off between clutch size and incubation efficiency in the Barn Swallow Hirundo rustica

Abstract

Summary To investigate reproductive costs associated specifically with incubation for a single‐sex 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. 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. 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. 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. 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. 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.