Colony dynamics and social attraction in black-fronted terns, Chlidonias albostriatus

Abstract

Black-fronted terns (Chlidonias albostriatus) are one of six endemic bird species that rely on New Zealand’s braided river ecosystems for breeding. Black-fronted terns have a small, declining population and are classified as globally endangered, primarily due to predation. Unlike many other endangered species in New Zealand, black-fronted terns cannot be translocated to offshore, predator-free islands as braided river habitat exists only on the mainland. Currently, predator control, at varying scales, and habitat enhancement are the primary management strategies for black-fronted terns, neither of which have proven more than locally effective at reversing current population declines. Effective black-fronted tern management is challenging, not only due to the dynamic and unpredictable nature of the braided river environment, but also the behaviour of the terns themselves. Black-fronted terns frequently change their breeding colony locations both within and between years. The current research aimed to investigate black-fronted tern colony dynamics, and determine the viability of social attractants as a tool for black-fronted tern conservation. The location and size of black-fronted tern breeding colonies have been recorded from braided river bird surveys conducted over 13 years (2004-2015). Black-fronted terns are believed to have low site-fidelity due to the instability of their breeding habitat, small colony sizes and exposure to high predation rates. Two out of nine rivers analysed had colony distributions significantly different to random, a further two rivers had significant clustering of colony locations. Although the clustering was only significant in two rivers, the trend of clustering was consistent across all rivers analysed. There was no overall trend between colony size and the proximity of colonies in the previous or following seasons. Overall, these results support our a priori hypothesis of low site-fidelity in black-fronted terns, although, consistent clustering and spatial distribution trends suggest that they may exhibit greater fidelity to sites which remain suitable. Social attractants, decoys and audio playback, were deployed at ten sites within nine Canterbury braided rivers in the 2016 breeding season. We found that the terns interacted significantly (P < 0.001) more with the social attractants compared to the control plots (social attractants absent). Differences in tern interactions observed could not be explained by the differences in habitat between the experimental plots. Nearest tern breeding was recorded for eight of the ten sites, with five of these nesting records occurring within 300 m of the experimental plots. These results suggest that social attraction has the potential for use in black-fronted tern conservation. However, further research is required to determine the most attractive social attractant set up and whether the attractants can influence tern breeding colony locations. Camera traps were also trialled, recording tern behaviour at the experimental plots. At this stage, camera traps are not recommended as a replacement for human observation.