Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Christopher G. Guglielmo


Among vertebrates, performance of juveniles is typically poor compared to adults. During fall, North American passerine birds migrate south ahead of deteriorating conditions on northern breeding areas. During this migration, juvenile passerines are typically three or four months old and have predictably poor performances. Differential performance could be due to lack of experience, selection against poor-performing cohort members, or physiological constraints of juveniles. Limited evidence suggests that physiology of juveniles may differ from adults during fall migration. I investigated how body composition, metabolic rate, and digestive physiology of juvenile and adult passerines differ during fall migration. In Chapter Two, I compared lean mass and fat mass measured by quantitative magnetic resonance, and dry masses of organs and muscles from salvaged carcasses of juvenile and adult migrants from four passerine species. In general, juveniles had more lean mass and heavier digestive organs (especially liver) than adults among hermit thrushes (Catharus guttatus), Swainson’s thrushes (Catharus ustulatus), ovenbirds (Seiurus aurocapilla), and white-throated sparrows (Zonotrichia albicollis). Principal components analysis of all organs and muscles revealed that juveniles for three of four species had overall larger digestive components and smaller flight muscles than adults. In Chapter Three, I used open-flow respirometry to measure basal metabolic rates (BMR) of juveniles and adults from two passerine species captured during fall migration at Long Point, Ontario. Controlling for body mass, juveniles had higher BMR than adults in both Swainson’s thrushes and white-throated sparrows. In Chapter Four I conducted short-term total collection mass balance feeding trials with Swainson’s thrushes and white-throated sparrows captured during fall migration stopover at Long Point, Ontario. Juvenile thrushes used higher food intake rather than greater utilization efficiency to achieve higher metabolizable energy intake than adults. Both age-classes of white-throated sparrows had similar measures of digestive physiology. Evidence suggested no difference in mean retention time between age-classes, despite larger guts of juveniles. Heavier digestive organs likely contributed to higher BMR of juveniles and allowed juvenile thrushes to consume more food during feeding trials. I propose that age-related differences in foraging ecology and diet composition are responsible for larger digestive organs of juvenile passerine migrants.