Smith, Adam D. – Exploring raptor migration using stable isotope analysis: The Northern Goshawk in western North America. 2006.
Chapter 1: Stable-Hydrogen Isotope Variation in Northern Goshawk
The analysis of stable-hydrogen isotope ratios in feathers (dDf) allows researchers to investigate avian movements and distributions to an extent never before possible. Nonetheless, natural variation in dDf is poorly understood and, in particular, its implications for predictive models based on stable-hydrogen isotopes remain unclear. I employed hierarchical linear modeling to explore multiple levels of variation in the stable-hydrogen isotope composition of Northern Goshawk (Accipiter gentilis) feathers. I examined (1) inter-individual variation among goshawks from the same nest, and (2) intra-individual variation between multiple feathers from the same individual. Additionally, I assessed the importance of several factors (e.g., geographic location, climate, age and sex characteristics) in explaining variation in dDf. Variation among individuals was nearly eight times the magnitude of variation within an individual, although age differences explained most of this inter-individual variation. In contrast, most variation in dDf values between multiple feathers from an individual remained unexplained. Additionally, I suggest temporal patterns of dD in precipitation (dDp) as a potential explanation for the geographic variability in age-related differences that has heretofore precluded the description of movement patterns of adult raptors using dDf. Furthermore, intra-individual variability necessitates consistency in feather selection and careful interpretation of Drbased models incorporating multiple feather types. Finally, although useful for describing the movements of groups of individuals, I suggest that variability inherent to environmental and intra-individual patterns dDp and dDf respectively, precludes the use of stable-hydrogen isotopes to describe movements of individual birds.
Chapter 2: Patterns of Northern Goshawk Migration in Western North america: An Application of Stable Isotopes
The fall movements of Northern Goshawks (Accipiter gentilis; hereafter goshawks) are not well understood in North America, complicating the interpretation of trends from counts at migration monitoring stations (MMSs). The use of stable isotope markers in feathers provides the potential to describe patterns of avian movements and distributions without the limitations of conventional markers. I used stable isotopes of hydrogen and carbon in feathers (dDf and d13Cf, respectively) to describe the origins of goshawks captured at five MMSs in the western United States. Additionally, I examined geographic variability in patterns of goshawk migration and documented patterns of sexrelated differential migration. In both nestling and adult goshawks, a strong relationship existed between dDf and the dD of local precipitation; this represents the first study to document such a relationship in adult raptors. Variation rendered d13C ineffective for describing goshawk origins or movements. Migrant samples from all five MMSs contained goshawks originating from local source areas (i.e., within 500 kin); additionally, migrants from several MMSs likely originated from regional source areas (i.e., within 500 1000 km of the MMS). Patterns of dDf indicate that the fall movements of juvenile goshawks are multidirectional and adaptable. Additionally, dDf indicated sex-related differences in patterns of migration, but the complexity of goshawk movements precluded any specific descriptions of differential migration in juvenile goshawks. With adequate sample sizes, stable isotopes can identify absolute shifts or changes in the relative contribution of source areas throughout the migration season. Furthermore, stable isotopes can help identify sex-related patterns of migration when common methodology fails. Several methodological and biological challenges need to be resolved before dDf realizes its full potential for describing avian origins and movements in western North America.