BRANTA — Yaara Aharon-Rotman
Challenges within the annual cycle of long-distance migratory waders along the East Asian-Australasian Flyway
Institution: Deakin University, Melbourne, Australia
Supervisors: Marcel Klaassen & Bill Buttemer
Subject Keywords:East Asian-Australasian Flyway, migratory stopover sites, migratory waders, threatened
Species Keywords: migratory waders
Of all flyways in the world, the East Asian-Australasian Flyway (EAAF) has the highest number of threatened waterbird species; the decline in many of these species being mostly attributed to habitat loss and deterioration. However, besides major perturbations of migratory stopover sites along the E and SE Asian coast, it cannot be denied that conflicts with man are also on the increase in their Australian wintering grounds (their main non-breeding grounds) and that climate change is having a notably impact on their Arctic breeding grounds.
Adopting a somewhat holistic approach, this thesis investigates the challenges that migratory waders (or ‘shorebirds’) face along the EAAF over the course of their annual cycle. Using data from shorebirds caught at different Australian wintering areas (i.e. King Island [Tasmania], SE Australia [South Australia and Victoria] and Broome [NW Australia]) and the utilisation of a range of techniques such as assessment of age-composition, stress biomarkers, parasite infection rates, tracking devices and modelling, we assessed the various challenges the birds are facing and made predictions on their potential capacity to cope with future changes.
In Chapter 2, using juvenile percentages assessed on the shorebirds’ migratory staging and wintering sites, we show that the breeding success of high Arctic nesting waders is primarily influenced by summer temperatures in the Arctic. Moreover, we demonstrate that weakening lemming cycles, which had an indirect effect on breeding success of waders and geese in the past, have triggered a change from once cyclical breeding success in waders from the EAAF as well as from the East Atlantic Flyway. As there was no obvious decreasing trend in the breeding success of waders, the downward trend observed for many species is assumed to be a result of deteriorating conditions at stopover and/or wintering sites.
The rate of fuel deposition importantly determines a migrant’s speed and success of migration. In Chapter 3 we show that fuelling rates are correlated with latitude, increasing from the tropics to the poles. In accordance with the “time minimization hypothesis”, this latitudinal trend has an effect on the migration strategy of birds flying from Australia to their Arctic breeding grounds. They typically fly in one go, across the equator, skipping low quality habitats in the tropics to arrive at high quality sites at higher latitudes, making small hops from thereon. This long non-stop journey requires appropriate preparation and it therefore highlights the disproportional reliance of long distance migratory waders on sites at high latitudes (i.e. S Australia and temperate environments in the N Hemisphere). Stochastic dynamic modelling predictions (Chapter 4) suggested that birds that failed to fuel sufficiently at a high latitude wintering site in South Australia, may be severely disadvantaged with respect to survival and reproductive success.
Choosing a wintering site is an important decision for migratory birds, as birds in favourable locations have higher survival and possibly even a better breeding success in the following summer. By comparing three different wintering habitats at different latitudes using physiological tools (stress biomarkers and parasite infection rates), we find further support for the high benefits of wintering at high latitudes (Chapter 5). Birds wintering in South Australia and Tasmania (35-40°S) were found to have better health indices than birds wintering in Broome, NW Australia (18°S). The use of physiological tools provide a useful means for assessing habitat quality in terms of the physiological effects on the animal itself, and non-invasive tools are increasingly being used for this task. A validation study presented in this thesis (Chapter 6) confirms the validity of using feathers to measure stress hormones in birds. However, it also emphasises the need for caution in the interpretation of such data, in part because of individual variation.