Analysis - waterbird numbers v rainfall

The monthly survey of waterbirds on Curl Curl Lagoon has been going for the best part of 8 years. The data has been kept in a (large) spreadsheet, with about 90 data points for each species. With so much data, we have been hopeful that we could could identify the factor, or factors, that would explain the wide variation in numbers from month to month and year to year. We knew that in February 2020 there was a sudden fall-off in numbers, particularly amongst the waterfowl - mainly ducks and coots. Numbers of wading birds such as White-face Herons and Royal Spoonbills, while always low, remained more or less steady. The drought that started in 2017 broke throughout NSW in early February 2020. It seemed clear that the reduction in total waterfowl from around 140 per monthly survey to around 60 resulted from a mass exodus from the coastal lagoons to newly replenished inland waterways. The drop was especially pronounced for Eurasian Coots and Hardheads, the most common species throughout 2017. Hardheads disappeared completely and only minuscule numbers have been seen since.

Based just on the 2020 event, our theory was, roughly, "dry in the west, waterfowl flock to coast, wet out west, birds leave coast". To test the theory, we graphed average monthly waterfowl numbers against total inland annual rainfall taking rainfall data for Dubbo Airport as an indicator of inland rainfall generally - see below.

The 2017 - 2019 drought is obvious, and although there has been some variation in annual rainfall since, compared to the drought years, the inland has remained wet and we could have expected that bird numbers on the lagoon would remain low. That was the case for 2022, although that result may have been affected by survey difficulties as a result of the retrictions imposed by the COVID epidemic. Otherwise, numbers have remained at around 80% of those in the drought years, disappointingly in contravention of our theory. Perhaps we need to wait for another drought to see whether there is a boost to bird numbers on the coastal lagoons.