Abstract
Variation in the diet of marine predators such as seabirds can be used to track environmentally-driven changes in ocean ecosystems. However, studies of predator diet must account for intrinsic influences on prey selection, such as changing nutritional requirements during breeding. Using digital photography, we investigated how the type and size of prey brought back to the colony by greater crested terns (Thalasseus bergii) changed in relation to breeding stage, and to variation in oceanographic conditions around Montague Island, Australia (36°15’S, 150°13’E). 2469 prey items were identified to species or family level over 35 consecutive days of photo-sampling in 2018. Australian anchovy (Engraulis australis), a surface-schooling clupeid fish, was the most abundant prey returned to the colony during all breeding stages (84.5%). The proportion of anchovy increased from 77.0% when birds were provisioning their adult partners during incubation, to 92.4% when they were provisioning chicks, suggesting selective foraging behaviour on this energy-dense species as a means to facilitate rapid chick growth. Anchovy size was largest during incubation (91.1 ± 14.9 mm), smallest during early chick provisioning (71.8 ± 11.0 mm), and increased slightly during mid-provisioning (79.6 ± 11.9 mm), indicating adaptive prey selection that is matched to the physical requirements of different breeding stages. The proportion of anchovy prey was also influenced by extrinsic environmental factors, with anchovy becoming more dominant with increasing local sea surface temperatures, up to ~17.5°C. Our findings highlight the importance of examining both intrinsic and extrinsic determinants of diet composition across breeding stages in seabird populations.
Foraging behaviour and habitat use by brown skuas Stercorarius lonnbergi breeding at South Georgia
