Black Noddy is a small, colonial seabird that inhabits tropical and subtropical regions of the Pacific and Atlantic oceans. It feeds predominantly on small fish, the birds often relying on larger predatory fish to drive shoals of small fish to the surface where they can be caught. In Australia, Black Noddies breed primarily on islands within the Great Barrier Reef. Islands in the Capricornia group in the southern reef offshore of Gladstone are particularly important, supporting a combined population of around 302,000 breeding pairs. Black Noddies nest in summer, from October to February, with the pair sharing all nesting duties. The nest is a platform of leaves draped over a horizontal branch of a tree with a shallow depression in which a single egg is laid. Once the fluffy black chick hatches, the pair take turns foraging, returning to regurgitate food for the chick.
To get a better understanding of noddy foraging behaviour, PhD student Nick James supervised by Graeme Cumming recently fitted GPS tracking devices weighing less than 3 grams on chick-rearing adult noddies on Heron Island. Since the birds are so tame and confiding, they were caught by hand on the nest. While the initial capture usually went smoothly, recapturing them several days later to retrieve the devices and download the tracking data was trickier, usually requiring a silent approach in the pitch dark at night. The growth and survival of the chicks was also monitored to link the foraging behaviours of the adults to chick growth and survival.
The first three years of the study observed some variation in food availability that affected the distances that the birds had to travel to find food. In a good year (2021, light blue lines in Figure 1), the birds were able to forage close to the island to get enough food, whereas in a bad year (2019, dark blue lines) they had to travel further away. Over this period, the average foraging trip length was 71.5 km with the birds spending 18 minutes away from the colony on average. In the final year of the study in 2022, conditions changed dramatically for the worse. The average foraging trip distance blew out to 607 km (maximum 1057 km) with the birds spending 45 hours away from the colony on average as they travelled far to the north, some using Bushy Island as an overnight roost while they foraged over the north-eastern reefs (Figure 1). Adults returned from these marathon trips with less food, which significantly reduced chick growth. Also, because the adults were away from the colony for such extended periods, chicks were left exposed to the elements and predators for longer. Consequently, chick mortality increased from very low levels to 54-64%. Hungry, unattended chicks that fall out the nest to the ground become ‘runners’, doomed to a slow death by starvation or predation because the adults do not recognize chicks out the nest. Eastern Reef Egrets and a recently established population of Silver Gulls were well fed on noddy chicks over this time.
The reduced food availability for Black Noddies was linked to warmer sea surface temperatures. Mass mortalities of Black Noddy adults and chicks due to apparent starvation has occurred at Heron Island in earlier years, also coinciding with elevated sea surface temperatures and coral bleaching events. While populations do fluctuate with natural climate variability, these research findings highlight the vulnerability of breeding seabirds to rising sea temperatures because of climate change. These changes can decouple breeding colony locations from favorable food resources, forcing birds to travel much further to find food.
REFERENCES
- Dyer, P.K. et al. 2005. Breeding numbers and population trends of Wedge-tailed Shearwater and Black Noddy in the Capricornia Cays, southern Great Barrier Reef. Emu 105: 249-257.
- Higgins, P.J. et al. 2006. Handbook of Australian, New Zealand and Antarctic Birds (HANZAB).
- James, N.L. & Cumming, G.S. (2024). Climate change may impact habitat complementation and cause disassociation for mobile species. Landscape Ecology 39: 139.
