Time and Energy Budgets of the Willow Flycatcher (Empidonax traillii) during the Breeding Season

We quantified the time-activity budgets of Empidonax traillii in five phases of the nesting cycle during 2 yr and converted the results into energy budgets in order to learn the relative costs of reproductive events and their seasonal correlates. Salient results are: (1) Basal and thermostatic power consumption preempt an average of 43% of the total daily energy expenditure ($DEE_{tot}$) in males and 42% in females; variation of air temperature accounts for about 90% of variation in$DEE_{tot}$. (2)$DEE_{tot}$in males varies from 54 kJ/day (incubation) to 69 kJ/day (nestling phase) and in females from 52 kJ/day (incubation) to 65 kJ/day (prenesting). (3) The portion of$DEE_{tot}$allocated to facultative activities varies only moderately, in males from 31 kJ/day (prenesting) to 35 kJ/day (nest-construction phase) and in females from 28 kJ/day (incubation) to 36 kJ/day (prenesting, ovogenesis). (4) Energy allocated to maintenance, production, and facultative activity, respectively, tends to vary reciprocally, minimizing interphasic variations of$DEE_{tot}$. (5) We argue that the time budgets of both males and females contain a large fraction of uncommitted or "loafing" time. We interpret this as an expression of Wilson's "principle of stringency," which suggests that selection is episodic and that uncommitted time (and energy) serves as a buffer against unpredictable episodes of "stringency," such as inclement weather and/or food shortage. (6) The grand seasonal mean$DEE_{tot}$for males and females (54.5 kJ/day) is 1.6% less than an allometric prediction for free-living birds in general and falls between values for birds of the same body mass as E. traillii that feed on the wing (73.2 kJ/day, swallows and hummingbirds) and those that do not feed on the wing (46.9 kJ/day), differing significantly from the former (P < 0.05) but not the latter. The foraging routine of E. traillii entails no more flight time (and often less) than the routines of ground-foraging birds, and so$DEE_{tot}$is substantially less than that of truly aerial insectivores.