Parental Birds Incubating Larger Clutches Regulate Their Field Metabolic Rates in Response to Environmental Changes

Abstract

Species are capable of flexible energy allocation in response to changes in food availability. The extent to which individuals are able to adjust their energy allocation is presumed to be dependent on individual quality. Individuals of lower quality (i.e. better physiological condition) may be at an energy ceiling, and lack the reserves available to be able to expend more energy when environmental conditions deteriorate. Using the doubly labeled water method, we measured the field metabolic rates of free-ranging Black-tailed Gulls Larus crassirostris incubating two- and three-egg clutches during two consecutive years. Lower sea surface temperatures in 2013, compared with those in 2012, provided indirect evidence of poorer food availability in 2013 than in 2012. Parent gulls incubating three-egg clutches were able to increase their energy expenditure, in response to the poor food conditions in 2013, whereas those incubating two-egg clutches were not. Within a gull population we found heterogeneity of the energy allocation process in response to changes in food availability.