Avian assimilation and dispersal of carbon and nitrogen brought ashore by breeding Westland petrels (Procellaria westlandica): a stable isotope study

Abstract

Only a small proportion of the marine nutrients brought ashore by breeding colonial seabirds is retained in colony soils. However, means by which these marine nutrients can be dispersed to the wider terrestrial ecosystem have rarely been identified. In this study, moulted feathers from Hemiphaga novaeseelandiae (an endemic pigeon; n=9) and Gallirallus australis (an endemic flightless rail; n=3) and from a Procellaria westlandica (Westland petrel) colony were analysed for ¹³C and ¹⁵N enrichment. Potential diet items from both the colony and a control location were also analysed, and their contributions to the diets of H. novaeseelandiae and G. australis calculated. Hemiphaga novaeseelandiae assimilated marine N but not marine C, and showed a wide range in the significance of different diet sources. Food for most H. novaeseelandiae came from within the colony, implying minimal transport of marine nutrients to the wider terrestrial environment. All G. australis food came from within the colony, but 29–39% was from petrel tissue, or invertebrates feeding on petrel material. Thus, both H. novaeseelandiae and G. australis assimilated nutrients brought ashore by P. westlandica, but neither was shown to disperse the nutrients much beyond the petrel colony. However, birds that are now locally extinct may have dispersed marine nutrients more widely. Future research should focus on the consequences of restoring these species to petrel colonies as part of attempts to re-establish the functioning of pre-human ecosystems in New Zealand.