Using stress biomarkers to assess wintering habitat quality in migratory shorebirds

Background/Question/Methods

Migratory shorebird populations are under threat worldwide, but the vast range of locations they use annually makes it difficult to identify the key factors responsible for their decline. Ruddy Turnstones Arenaria interpres that use the East Asian-Australasian Flyway winter over large geographic range. Some individuals overwinter in north Western Australia and some continue to the southern coasts of Australia and Tasmania, further increasing their already substantial flight distances and migratory costs. Individuals residing in these wintering locations experience different environmental conditions, including differences in levels of anthropogenic disturbance, flight costs, food abundance, and threats from predators and pathogens. If variance in these conditions creates fitness inequalities, we would expect corresponding differences in levels of stress biomarkers between these wintering populations. We estimated chronic stress levels by measuring leukocyte profiles, feather corticosterone (CORT) content and natural antibody levels in three different wintering populations in Australia. We also quantified differences in predators, energy expenditure, flight costs and parasite loads at these sites to see whether the higher migration costs in flying further are offset by a better habitat. The combined measures of these stress biomarkers provide a means to test whether differences in habitat quality are contributing to differences in population stability.

Results/Conclusions

Although flight costs and daily energy expenditure were higher in birds overwintering in Tasmania and South Australia than in north Western Australia, predators disturbance and heterophil:lymphocyte ratios from blood smears were lower in these populations, suggesting lower stress levels and potentially indicating a benefit of travelling further and increasing flight costs. The correspondence between blood heterophil:lymphocyte ratios and wintering sites retained statistical significance when time after capture was used as a covariant in the model. Our data also demonstrate differences between the populations in the response to acute stress in terms of the movements of white blood cells into the peripheral blood, an adaptive response to a short term stressor. We will also present the first data from Australian birds quantifying the incorporation of CORT into feathers developed on the wintering ground as an indicator of chronic stress, as well as natural antibody levels. Our evidence to date is that the benefits of occupying more distant winter locations outweigh the higher energy costs in using these sites. This can create fitness inequalities which can shape the distribution and population-wide demography of migratory species.