Body condition of a small passerine bird: ultrasonic assessment and significance in overwinter survival

The fitness benefits of intraspecific variation in physiological attributes have rarely been measured. Body condition, defined as the current status of metabolic reserves relative to likely demands, has often been implicated in subsequent survival, but has proved difficult to assess reliably in the live animal. A technique for assessing body condition, in terms of the main protein reserve of small birds, is presented. Pectoralis muscle thickness was measured in live birds using ultrasound reflection from the sternum. The relationship between the relative size of pectoralis muscles in autumn and the likelihood of overwinter survival in the dipper Cinclus cinclus was examined. The pectoralis reserves of male dippers surviving overwinter were significantly greater than those of birds which died or disappeared between late November and the breeding season in April. In contrast, variation in autumn condition of females was unrelated to overwinter survival.