The life-history of short-tailed shearwaters (Puffinus tenuirostris) in response to spatio-temporal environmental variation
Creators: name: Vertigan, CA
Publisher: name: University Of Tasmania
Year: 2023

description: Ecological changes in the phenology and distribution of plants and animals mediated by changes in the environment are occurring across the globe and have been documented for a wide range of species. However, determining the specific mechanisms that affect the life-history parameters of organisms is typically difficult. Measuring population trends over time, and in relation to broad-scale environmental state, is one of the simplest ways of linking the effects of physical environmental variation to its direct effects on biological systems. This study is concerned with describing key life-history parameters and population demography of a wide-ranging and abundant pelagic seabird, the short-tailed shearwater (Puffinus tenuirostris). The population demography of this uppertrophic consumer is investigated directly within the context of its spatio-temporal use of the marine environment and its response to environmental variation. In this thesis I present data and analyses for: 1. Population trends of short-tailed shearwaters and the sympatrical/y breeding little penguin (Eudyptula minor) - Trends from a colony of shorttailed shearwaters at the southern most extent of their distribution were documented from 2003-1 O and found to be declining at a rate of 15% per annum. A series of hypotheses were constructed to provide possible explanations for the observed trends including source-sink population dynamics, recreational 'mutton-birding', by-catch from fisheries, investigator effects, changes in onshore habitat characteristics, and distal changes in food source availability. In contrast little penguins were found to be increasing at a rate of 17% per annum with their increase attributed to the removal of feral cats from Wedge Island in 2003. 2. The effects of investigator disturbance on short-tailed shearwaters and little penguins - Researcher activities were monitored during the course of the study to identify any possible effects on rate of egg laying, chick survival or chick handling. No effect of the investigator was observed in short-tailed shearwaters for any of life-history attributes measured with the effect of investigators on little penguins unable to be conclusively determined. 3. The spatio-temporal use of the Southern Ocean during the breeding season of short-tailed shearwaters - Geo-location devices were deployed on 20 birds in the breeding seasons of 2004-05 and 2007-08. Foraging locations and behaviours were identified over a temporal scale encompassing the entire breeding season from the pre-egg laying 'honeymoon' period, to chick rearing. This study determined that trip characteristics changed significantly over the duration of the breeding season. Bimodal foraging strategies occurred once chicks had hatched and the duration of long trips contracted from 20.1 ¬¨¬± 10.6 days to 14.6 ¬¨¬± 2.2 days. Foraging occurred in a range of water masses from the Australian continental shelf to the Polar Frontal Zone with use of Southern latitudes increasing over the course of the breeding season. 4. Identifying Annual variability in the diet of short-tailed shearwaters using stable isotope analysis - Whole blood samples were collected over 4 breeding seasons and used to identify trophic foraging level and latitudinal foraging location. Inter-annual differences in diet were observed during the four breeding seasons with i513C and i515N values in 2004-05 and 2005-06 significantly higher than in 2006-07 and 2007-08. Despite the contrast in diet between seasons, there was no direct relationship detected between prey type and reproductive performance indicating that short-tailed shearwaters are flexible in their response to changes in prey availability. 5. Influences of environmental variation on the population trend of short-tailed shearwaters- After identifying specific spatio-temporal regions of importance, this information was used to link physical oceanographic parameters such as sea surface temperature (SST), primary productivity (Chia), wind speed and direction, sea surface height (SSH), sea surface height anomaly (SSHa) and ice edge extent to the decline in breeding short-tailed shearwaters. While no relationships were determined between chick survival and environmental variables, we detected a relationship between a decline in egg laying and an increase in pre-egg laying sea surface height anomaly (SSHa) in the Southern Ocean. General discussion and recommendations for future research-Understanding the causal mechanisms driving changes in population demography is complex. Changes are often subtle and may be interconnected. By utilising multidimensional approaches including telemetry, diet and physical oceanography, improvements can be made in determining the links between the marine environment and upper trophic consumers. As environmental changes may affect different demographic parameters and the response variables may be direct ( i.e. an increase in adult mortality or a decrease in adults choosing to breed) or lagged (i.e. reductions in breeding success), it is likely that multiple processes are contributing to the decline of shearwaters on Wedge Island. This colony should continue to be monitored, in addition to others within the species distribution as only long-term data sets will determine if this is part of an oscillation in breeding numbers or a response to more long-lasting shifts in environmental conditions. Chick diets should be obtained in addition to adult diet corresponding to the preegg laying period. Marking of birds within the colony will assist in making stronger conclusions as to the effect of emigration. Further telemetry studies will aid in detecting inter-annual differences in foraging behaviours.