Metabolic rate of nocturnal incubation in female great tits Parus major in relation to clutch size measured in the natural environment

To study clutch size related energetic costs of incubation, clutch sizes were experimentally manipulated and metabolic rate of female great tits Parus major (L.) during nocturnal incubation (MRinc) was measured with the use of mobile oxygen analysers. Each individual was repeatedly measured, while incubating either reduced, control or enlarged clutches. In contrast to most previous studies, the experiment was performed in the field to put possible effects of the clutch size manipulation in perspective to other factors explaining variation in MRinc. Females spent on average 0.65 ± 0.09 J s-1 during first clutches in 2004 and 0.55 ± 0.07 J s-1 during second clutches in 2001 at mean ambient temperatures of 9.3 ± 2.4 ̊C, and 14.5 ± 2.4 ̊C, respectively. Females spent energy at a higher rate when incubating enlarged clutches (6 10%; three additional eggs), but did not spend significantly less energy when incubating reduced clutches as compared to controls. The effect of clutch size on MRinc was relatively small compared to that of ambient temperature, which explained most of the variation in MRinc (43 49% per 10 ̊C). Nest thickness and year (or breeding attempt) explained an additional part of the variation. MRinc exceeded that of basal or metabolic rate of non-incubating birds (2.0 times BMR, and 1.2 – 1.6 times MR8 ̊C, respectively). Comparison between species reveals that MRinc is relatively high for smaller bird species, which can be explained by the energetic costs of thermoregulation. The incubation phase may be a demanding period due to its timing in the season.