| Summary: | We used doubly labelled water to study the field metabolic rates of breeding Kittiwakes Rissa tridactyla during the incubation phase, and to compare these with the metabolic rates of Kittiwakes rearing chicks. During the incubation phase, birds with two eggs spent an average (± SE) of 915 ± 134 kJ day-1. This was similar to published estimates of Kittiwake energy expenditure during the chick-rearing phase, and similar to our own measurements of metabolic rates in a small number of birds rearing two chicks (863 ± 177 kJ day-1). Our results corroborate the current view that incubation is not a phase of low energy expenditure, even in a large bird like the Kittiwake, and even in a species where both parents incubate. There was high variability of energy expenditure between birds, and this was largely because birds spent varying proportions of their time on and off the nest and did not show simple diurnal cycles under the conditions of 24 hour daylight. Birds spent 559 ± 197 kJ day-1 while at the nest, and 1241 ± 154 kJ day-1 away. Males had higher energy expenditure than females, and this was because they spent more time off the nest, not because they were bigger. There was no evidence that metabolic rates were influenced by wind speed or temperature. In order to look at whether birds had to work harder to incubate larger clutches we placed an extra egg in some nests and measured metabolic rates of the adults. Those with an extra egg spent 1011 ± 163 kJ day-1 which was not significantly greater than the 915 kJ day-1 spent by those with their normal clutch size. link_to_subscribed_fulltext
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