Understanding the movements of migratory birds and connecting the different stages of their annual cycle is necessary for the conservation and management of migratory bird species. Stable isotope technology has the potential to shed light on the movements of migratory species and to help us better understand their population dynamics. Several studies use stable hydrogen isotopes in particular to predict origins of birds sampled during migration or in winter. However, recent work on stable hydrogen isotopes in feathers (δDf) draws into question the utility of this technology in estimating origins of migrants.
My objective was to determine whether stable hydrogen isotope analysis is a useful technique to estimate the origin of red-tailed hawks (Buteo jamaicensis) migrating through monitored migration sites in the western United States. I used the base map created by Lott and Smith (2006) to predict origins from the δDf values and looked for relationships between that estimated origins of migrants and year of capture, sex, and passage date. In addition, I compared δDf signatures with the signatures predicted by the base map for breeding locations of adult birds outfitted with satellite telemetry units and for a recaptured bird banded as a nestling. Predicted origins were frequently both to the north and south of migration monitoring stations and covered large areas. Identifiable migration patterns existed only at the Goshutes Mountains site where individuals from lower latitudes migrated through earlier than individuals from higher latitudes and during one season when females migrated through later than males. Smaller sample sizes may have contributed to a lack of patterns at the other migration sites. The satellite telemetry individuals and recaptured juvenile did not match up to predicted signatures in most cases.
The broad possible source areas for migrating red- tailed hawks in the fall rendered this technique ineffective for determining breeding locations or natal origins using my procedures. Furthermore, the concerns raised regarding using this technology for conservation and management suggest other techniques, such as satellite telemetry, may provide more reliable results.