Developmental stress reduces body condition across avian life-history stages: A comparison of quantitative magnetic resonance data and condition indices.

Animals exposed to stressful developmental conditions can experience sustained physiological, behavioral, and fitness effects. While extensive research shows how developmental stress affects development, few studies have examined the effects on body composition. To test the effects of developmental stress on nestling and adult body composition, we dosed nestling zebra finches (Taeniopygia guttata) with either a corticosterone (CORT) or control treatment. We calculated condition indices (scaled mass, residual mass, and ratio indices) from morphometric measurements and used quantitative magnetic resonance (QMR) to assess body composition during early development and adulthood. We compared these three traditionally-used condition indices to QMR-derived body composition measurements, to test how well they predict relative fat mass. Our results show that developmental stress decreases body mass, and has a dose-dependent effect on tarsus length in nestling birds. Furthermore, stress treatment during the nestling period had long-lasting effects on adult body mass, lean mass and tarsus length. None of the three condition indices were good indicators of relative fat mass in nestlings, but all indices were closely associated with relative fat mass in adults. The scaled mass index was more closely associated with relative fat mass than the other condition indices, when calculated from wing chord length in nestlings. In adults however, the residual mass index and the ratio index were better indicators of relative body fat than the scaled mass index, when calculated from tarsus length. Our data demonstrate the short and long-term impact of developmental stress on birds, and highlight important age-related factors to consider when using condition indices.