Since the arrival of humans, New Zealand has lost over 90% of wetlands as a direct result of conversion of the land for urbanization and farming. The Matuku/Australasian bittern (Botaurus poiciloptilus), a native wetland species, has undergone significant population declines and is currently classified as 'Threatened-Nationally Critical" and is at risk of extinction. Recent autopsies have found contemporary bitterns have depleted bone marrow fats, suggesting the birds are undergoing prolonged periods of nutritional stress, possibly due to the decline of their native prey. In this thesis, I used ptilochronology and stable isotope analyses on feathers collected from bitterns spanning in dates over the past ~130 years and from several regions of New Zealand to assess the prevalence of nutritional stress. I used feathers from both contemporary birds and historical birds, sourced from museum skins. I first conducted a ptilochronology analysis, the process of measuring growth bars on feathers which can provide a direct index of nutritional stress in birds. I used 4 types of feathers and found a correlation between year and variance in growth bar lengths, suggesting the diets of contemporary birds has become more variable and they are experiencing more nutritional stress than their historical counterparts. I next performed stable isotope analysis, specifically analysing the nitrogen isotopic values to identify trophic position change and carbon isotopic values to identify environmental change. I used baseline samples collected from 2 regions to rule out the influence of environmental condition on the bittern isotopic values. I first conducted stable isotope analysis on breast feathers and found both geographical variation between 4 regions of New Zealand and temporal variation over the past ~130 years. I then performed stable isotope analysis on the primary feathers of birds from the Canterbury and Bay of Plenty regions by repeated sampling along the lengths of feathers. This allowed me to identify inter- and intra-individual variation in the isotopic values of bitterns within and between regions. Finally, I then compared the stable isotope values of sections to their corresponding growth bar data but found no correlation. My research has identified that contemporary bitterns are experiencing higher levels of nutritional stress compared to historical birds, however more research is needed to understand the cause behind the isotopic variation observed within and between populations to identify which regions should be the most important for focusing conservation efforts.