Age-related changes in efficiency among breeding Common Terns Sterna hirundo:: measurement of energy expenditure using doubly-labelled water

We used doubly-labelled water to measure field metabolic rates of breeding Common Terns Sterna hirundo, and hence to investigate differences in reproductive efficiency among birds of different ages. Field metabolic rates, activity patterns, changes in body mass, chick provisioning rates, and chick growth rates were measured for 14 males of known age (6-24 yr) during a 48-h period immediately after hatching of the first chick, when males provide most of the food. Males expended energy at high rates (3.3-6.3 x estimated basal metabolic rates). We found no significant relationship between male age and energy expenditure. Some aspects of male performance (feeding rates and food provisioning rates) were age-related due to changes in efficiency. Efficiency in the use of time and energy was low in the youngest breeding males, increased thereafter to peak at about 12 years of age, but appeared to decline again amongst the oldest breeding males. Changes in body mass and measures of activity were not correlated with energy expenditure and hence were not appropriate measures of reproductive effort.