Flight cost of a small passerine measured using doubly labeled water: implications for energetics studies

-The metabolic cost of rest and activity (cm3 CO2 g-1 h-') was measured for the European Robin (Erithacus rubecula, 18.6 g) at temperatures (Ta, C) from -15 to +30?C. Regressions expressing these costs are: night resting (MrN) = 4.23 0.0733Ta, day resting (MrD) = 5.10 0.0807T., hopping (MhOP) = 8.60 0.1256Ta. Daily energy expenditure (DEE) for robins held in an outdoor aviary was 64.8 kJ/day (SD = 9.2, n = 6), determined using the doubly labeled water technique. This was positively related to time spent in flight (tflY, h) such that DEE = 50.9 + 23.4ti,y. Flight cost for robins was estimated as 25.6 kJ/h (SD = 5.0, n = 6). This flight cost is about twice that predicted by various allometric equations. Robin flights in the aviary were short (3 m) and brief (0.78 s), indicating a mean flight speed (3.85 m/s) that was lower than the theoretical minimum power velocity (5.86 m/s). The European Robin has a relatively high wing loading (0.263 g/cm2) and aspect ratio (7.33). In a small bird with flapping flight these characteristics imply a high cost, particularly at low flight speeds. The high cost of flight was offset by its short duration. During 30-min observation periods, an average of 100.2 s was spent in flight, implying a sustained energy demand of only 3.04 kJ/h (2.7 x basal metabolic rate). The exceptionally high flight cost reported here (23 x basal metabolic rate) may be typical of short, brief aerial forays. Other doubly labeled water studies reveal a positive correlation between the time spent in flight and DEE, indicating its dominant impact on energy turnover in some free-living birds. Received 17 April 1985, accepted 19