THE DEMANDS OF INCUBATION AND AVIAN CLUTCH SIZE

We reviewed information on the demands of incubation to examine whethe r these could influence the optimal clutch size of birds. The results indicate that appreciable metabolic costs of incubation commonly exist , and that the incubation of enlarged clutches can impose penalties on birds. In 23 studies on 19 species, incubation metabolic rate (IMR) w as not elevated above the metabolic rate of resting non-incubating bir ds (RMR), but contrary to the physiological predictions of King and ot hers, IMR was greater than RMR in 15 studies on 15 species. Across spe cies, IMR was substantially above basal metabolic rate (BMR), averagin g 1.606 x BMR. Of six studies on three species performed under thermo- neutral conditions, none found IMR to be in excess of RMR. IMRs measur ed exclusively within the thermo-neutral zone averaged only 1.08 x BMR contrasting with the significantly higher figure of 1.72 x BMR under wider conditions. 16 of 17 studies on procellariiforms found IMR below RMR, indicating a significant difference between this and other order s. We could find no other taxonomic, or ecological factors which had c lear effects on IMR. Where clutch size was adjusted experimentally dur ing incubation, larger clutches were associated with: significantly lo wer percentage hatching success in 11 of 19 studies; longer incubation periods in eight of ten studies; greater loss of adult body condition in two of five studies; and higher adult energy expenditure in eight of nine studies. Given that incubation does involve metabolic costs an d given that the demands of incubation increase sufficiently with clut ch size to affect breeding performance, we propose that the optimal cl utch size of birds may in part by shaped by the number of eggs the par ents can afford to incubate.