Two methods to estimate the population abundance of the Silver Gull ( Chroicocephalus novaehollandiae ) – an enduring classic versus a rising star

We compared breeding pair estimates for the Silver Gull (Chroicocephalus novaehollandiae) near aquaculture operations using two methods: (1) quadrat-based ground surveys and (2) drone-assisted surveys. Annual estimates derived from each method varied by −15.9% to 21.6%. Interannual trends in breeding pairs were consistent between the methods, with both detecting declines from 2021 to 2023, and estimates ranging from 14,800 to 23,600, indicating regional population stabilisation since 2006 (20,750 breeding pairs). Cross-validation of nest count data obtained from each method indicated a significant, positive, linear relationship in nest-density estimates between methods at each island surveyed (Sibsey Island: r2 = 0.35; p = 0.03; Rabbit Island: r2 = 0.83; p < 0.001), with regression coefficients not differing from 1, indicating no systematic bias. Drone-derived colony area estimates were larger on four of the six occasions, with differences ranging from −17.3% to +57.4%, likely reflecting perimeter estimation error by field staff using handheld GPS during ground surveys. The strength of quadrat-based ground surveys undertaken is that they provided a comparable method from which temporal trends in Silver Gull numbers could be assessed. Quadrat-based surveys also required less field time (one survey), minimal training and equipment and enabled collection of biological data. Yet they were potentially disruptive to breeding gulls and subject to observer bias. In contrast, drone-assisted surveys required greater field time (two surveys), technical expertise and investment and although potentially constrained by vegetation cover and the need for precisely timed surveys, were likely less error-prone and conducted remotely with reduced disturbance to breeding gulls.