Energetic constraints on avian incubation : studies of three passerine species
Field studies were conducted with "three species of passerine, in order to investigate the possibility that an energetic constraint limits reproduction during incubation. Swallows (Hirundo rustiea), Dippers (Cinelus einelus) and Great Tits (Parus nlajor) were studied at sites in Central Scotland. All three species exhibit gynelateral intermittent incubation, so time and energy must be allocated between the conflicting demands of reproduction and selfmaintenance. An assessment of incubation ability in the Swallow was conducted by the manipulation of clutch size during incubation. There was evidence of a clutch size dependent cost, as the duration of the incubation period was prolonged for enlarged (15.6d) compared to reduced (14.8d) clutches. The proportion of eggs hatching successfully was also lower in enlarged (81 %) than in reduced (92%) clutches, though enlarged clutches still produced the greatest number of hatched young. Clutch manipulation did not influence patterns of nest attendance, or female body condition. No effects of incubation effort were detected posthatch on either parents or offspring. The effects of clutch size on field metabolism during incubation were investigated in the Dipper. Clutch size was manipulated and energy use measured by means of the doubly labelled water technique. The results were combined with previous data collected for incubating Dippers. The field metabolic rate of 33 incubating females averaged 5.41 ± 1.34 cm3 CO2 g-l h-1 , equivalent to a daily energy expenditure of 211.52" ± 51.25kJ ind-1 d-1 , e.3 times the basal metabolic rate. Clutch enlargement resulted in an increase in energy use to 4-6 times basal metabolism for some birds," but not for others. While the mean energy use did not differ between groups, the variation amongst birds was significantly greater for enlarged than control clutches. Energy use was also influenced by river flow rates, the duration of incubation sessions and behaviour during incubation recesses. Manipulation of the energy budget of incubating Great Tits was achieved by the reduction of thermoregulatory demands. Treated nest boxes were supplied with additional heat during the hours of darkness, resulting in an elevation of nest air temperature of e.4°C above the corresponding temperature for a control group, lasting for a period of 9 hours. This produced an estimated energetic saving of 10kJ per night. Heated birds increased the duration of both the ~period of continuous incubation overnight and of incubation sessions throughout the following day, resulting in an additional 51 minutes per day spent incubating compared to the control group. The metabolic rate of22 incubating Great Tits was 7.79 ± 2.43 cm3 C02 g-1 h- 1 , or 106.4 ± 32.2 kJ ind-1 d- 1 , equivalent to e.3 times basal metabolism. Energy use escalated for control, but not for heated birds at low ambient temperatures. The importance of reserve storage and utilisation, and of provisioning by the mate were evaluated in each species. A combined hypothesis was proposed to account for body condition during incubation, incorporating elements of programmed reserve utilisation, mass adjustment, maintenance of an insurance reserve and reproductive stress. In summary, the study found "evidence of an energetic constraint acting during incubation in these species." Energy use increased in a probabilistic manner with increasing clutch size, such that birds with large clutches increased their risk of being unable to incubate the entire clutch successfully. It was suggested that such a constraint could contribute to the determination of an upper limit for avian clutch size.