METABOLIC RATES AND BODY-MASS CHANGES IN COMMON POORWILLS (PHALAENOPTILUS-NUTTALLII, CAPRIMULGIDAE)

We studied field metabolic rate (FMR) and mass changes in nesting comm on poorwills (Phalaenoptilus nuttallii; Caprimulgidae) using the doubl y labelled water method. In June when poorwills were incubating a firs t clutch, FMR for females was significantly lower than for males (fema les: 28.3+/-3.0 kJ . d-1; males 85.0+/-12.7 kJ . d(-1)), but FMR did n ot differ between the sexes in August (59.6+/-8.7 kJ . d(-1)) when bir ds were incubating a second clutch. Mass change (MC) was significantly correlated with FMR (MC = 1.548 - 0.031 . FMR) and birds achieved mas s stability at FMR of 49.9 kJ . d(-1). The mass stable FMR of 49.9 kJ . d(-1) is 3.0 x BMR (basal metabolic rare) and is only 54% of Nagy's (1987; Ecological Monographs, 57: 111-128) allometric equation predict ing FMR for non-passerines. These data support the idea that low BMR, characteristic of poowills, results in low FMR. An analysis of mass ch ange as a function of foraging and FMR suggests that poorwills forage at a maximum rate of 200-300 sallies . night(-1) and that mass change results from fat deposition or mobilization. When vigorously calling t o defend territories in June, male poorwills have FMR of 5.1 X BMR, wh ich ranks in the top 5% of FMR recorded for birds. We suggest that mal es invest heavily in territorial displays and run a negative energy ba lance through June, leading to observed mass loss.