Meta-analysis of status and trends in breeding populations of black-fronted terns (Chlidonias albostriatus) 1962

Black-fronted tern (Chlidonias albostriatus) breeding populations on braided rivers in the South Island, New Zealand, are assumed to be in decline as their habitat comes under increasing pressure from exotic pests, hydroelectric power development and water abstraction. We collated 326 index counts of black-fronted terns from 2313 km of surveys on 84 rivers throughout their breeding range to test this assumption. Black-fronted terns were observed on 73% (n = 61) of rivers surveyed, and the sum of the most recent counts was 8325 birds. However, >200 black-fronted terns were counted on only 14% of rivers. We used generalised linear modelling to assess whether population trends could be detected using data from 29 rivers where counts were repeated 4–18 times between 1962 and 2008. We detected significant declines on eight rivers (range 5.5–15.8% p.a.), a significant increase on one river (Eglinton; 16.3% p.a.) and no trends on the remaining 20 rivers. The Eglinton River is the only site at which sustained predator control (aimed at mustelids) occurred throughout the monitoring period. Rivers on which declines have occurred are characterised by having relatively low flows (<30 m³ s^–1). At such rates, populations on low-flow rivers (64% of rivers surveyed, representing 51.4% of black-fronted terns counted on the oldest counts) would decline by a further c. 90% within 25 years. Based on these results we predict that if flows were reduced significantly on higher-flow rivers, rates of population decline would accelerate. We conclude that the IUCN status of ‘Endangered’ is appropriate for black-fronted terns, based on a predicted population reduction of around 50% over the next three generations (c. 30 years). This conclusion is supported by previous studies that described significant loss of habitat, low breeding success and vulnerability to predation by introduced mammals, and population models that predict continued declines towards extinction if management aimed at recovering populations is not instigated with some urgency.