Year-round diet, niche breadth, distribution and habitat use of a small procellariiform, the wedgetailed shearwater Ardenna pacifica breeding in south-eastern Australia

Animal distribution, movement and preferences in oceanic habitats respond to dynamic changes in their environment, they will adapt their foraging strategies which can impact predator-prey relationships, displace other species, change species range or change the timing of their annual cycle. Species such as procellarid seabirds are long-lived with low fecundity rates, relatively low numbers of offspring and extended provisioning periods, they balance survival with reproductive effort, known as k selected species.
Wedge-tailed shearwaters Ardenna pacifica like all seabirds are k selected species, who are a migratory procellariiform found throughout the Pacific and Indian Oceans, using a variety of tropical and sub-tropical habitats and breeding across a wide latitudinal range that recently has been extending southward. Information on their habitat use and niche breadth has not been well-documented for populations at their southern-most range, with limited information of this species from two different climatic zones in south-eastern Australia poorly understood.
This study compared the niche breadth, diet and habitat use of wedge-tailed shearwater populations located in two distinct climatic zones, a temperate and sub-tropical zone of south-eastern Australia, over a three-year period. Complementary methods were used in the analysis, namely direct diet samples coupled with bulk stable isotopic analysis of feathers obtained from birds on Muttonbird Island Nature Reserve located in a sub-tropical zone and Montague Island Nature Reserve located in a temperate zone. Spatial analysis was obtained from light geolocator tracking devices fitted to birds on Muttonbird Island during 2015 to 2018 and Montague Island during 2016 to 2018.
The direct prey samples obtained in the first year of the project revealed individuals from the temperate population had a lower prey diversity (Shannon diversity index 0.867) having a higher relative proportion of cephalopods compared to individuals from the sub-tropical population (Shannon diversity index 1.231) who had a larger niche breadth that included a greater variety of prey items with fish having a greater relative proportion in their diet indicating a broader trophic niche. Adult isotopic ratios had little variation inter-annually between the two populations as these were obtained from feathers grown in the nonbreeding habitat. However, ratios obtained during the breeding phase from chicks were higher in δ¹³C and δ¹⁵N values compared to the adults. Indicating resource partitioning occurred between what adults consumed and what they fed their chicks, with chicks been provisioned a higher trophic prey source over a narrower niche space.
Analysis from the tracking data confirmed population-specific partitioning in the habitats they used during the breeding phase with a spatio-temporal overlap of 0–22% compared to 0–30% during the non-breeding phase. The temperate population was consistent in the habitats they utilised during the breeding phase, with a staging detour undertaken in all years just prior to their winter migration. This was not the case for the sub-tropical colony who slightly varied their habitat use and only used a staging detour in the first year of the study.
During their migration birds from both populations followed a similar path, departing in a northward direction to the Solomon Islands before tracking to the northeast above Papua New Guinea and Indonesia to areas in the Philippine Sea. Their winter habitat revealed two 10 core areas used in the Philippine Sea; south east Taiwan and north west of Indonesia. The results show a divergence in the timing of their arrival (± 17 days) and departure dates (± 22 days) with the sub-tropical population typically arriving and leaving two weeks earlier for both.
By combining multiple years of data on wedge-tailed shearwater diet, stable isotope analysis, habitat use and spatio-temporal data across two climatic zones this study has found wedge-tailed shearwaters have the capability to adapt to and switch habitat and prey resources showing a high degree of foraging plasticity. This study has revealed information on the breeding behaviour of this species at their southern-most range with subtle differences in habitat use, diet and trophic levels which could explain why the sub-tropical population may not be performing as well as temperate populations. It is recommended future monitoring of this species be undertaken to determine if their distribution is shifting southward.