The Energetic Significance of Huddling Behavior in Common Bushtits (Psaltriparus minimus)

The daily energetics of a very small passerine, the Common Bushtit (Psaltriparus minimus), were examined by measuring 24-h existence metabolism, activity, and nocturnal metabolic rate. Bushtits consumed 80% of their body mass (5.5 g) in mealworms per day at 20°C. A lower energy intake than this resulted in marked weight loss (0.5-0.8 g), slight hypothermia upon exposure to 10°C for 4 h (Tb= 38.6°C), and a higher diurnal activity level than well-fed birds. Birds housed in pairs or larger groups spent portions of the day perching together and spent all of the nocturnal period huddled in a tight mass. These birds were also less active diurnally than isolated individuals and had lower nocturnal metabolism; the metabolism of paired birds was 79% of that of single individuals at 20°C. The energy savings gained by the gregarious nature of this diminutive species, reflected by the diminished cost to an individual of being a member of a flock, may be integral to its survival during periods of food scarcity or inclement weather.