Field metabolic rates of Kittiwakes Rissa tridactyla during incubation andchick rearing

We used doubly labelled water to study the field metabolic rates of breedin g Kittiwakes Rissa tridactyla during the incubation phase, and to compare t hese with the metabolic rates of Kittiwakes rearing chicks. During the incu bation phase, birds with two eggs spent an average (+/- SE) of 915 +/- 134 kJ day(-1). This was similar to published estimates of Kittiwake energy exp enditure during the chick-rearing phase, and similar to our own measurement s 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 Kit tiwake, and even in a species where both parents incubate. There was high v ariability of energy expenditure between birds, and this was largely becaus e 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. B irds spent 559 +/- 197 kJ day(-1) while at the nest, and 1241 +/- 154 kJ da y(-1) away. Males had higher energy expenditure than females, and this was because they spent more time off the nest, not because they were big ger. T here 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 incuba te larger clutches we placed an extra egg in some nests and measured metabo lic 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 th ose with their normal clutch size.