On inferring population trends of mobile waterbirds from aerial transect surveys in variable environments

Abstract

Monitoring waterbird populations in Australia is challenging for reasons of counting logistics, and because population aggregation and dispersion can shift rapidly in response to large spatio-temporal variations in resource availability. The East Australian Waterbird survey has conducted annual, aerial, systematic counts of waterbirds over eastern Australia for almost 40 years. It was designed to monitor waterbird populations using design-based inference though for many species this form of inference appears inadequate in the face of these challenges. Here we develop a state-space model-based Bayesian approach that, in addition to explicitly incorporating process noise and observation uncertainty, uses random effects and rainfall-derived covariates to model the year-to-year variation in the proportion of the total (super) population that is present on surveyed wetlands, and available to be counted. We use this model-based approach to estimate the superpopulation size of 45 waterbird species annually, and model the rate of population increase as a function of antecedent rainfall. The results confirm the strong positive effect of antecedent rainfall on population growth rates for nearly all species, and illustrate that species respond to rainfall differently in terms of habitat use, which influences whether they are present on surveyed wetlands. For many species, the year-to-year variation in the estimated proportion of the population on surveyed wetlands is very high. The results have implications for making inferences on population trends from these data, with the ability to model the year-to-year sampling variation a key requirement before the rate of population increase can be estimated with any precision. This study illustrates how to progress this approach, and infers that under average rainfall conditions, the general trend is for estimated superpopulation rates of increase to be negative, though for only a few species is this occurring with strong belief.

Habitat use by nomadic ibis and spoonbills post-dispersal from breeding sites

“…We found seasonal differences in SNI habitat preferences in relation to MRVBF topographic values and ANAE aquatic ecosystem and NVIS vegetation classes, but CLUM land use class preferences were similar year-round (Supplementary Figs. [9][10][11][12]. SNI showed a distinct preference for large depositional basins in summer (and to some extent spring), but a wider range of MRVBF zones used in autumn and winter, including valley oor environments and small depositional basins.…”

“…Large aggregate-nesting wading waterbird breeding events and locations in Australia have been relatively well studied and the focus of signi cant management effort, due to the national and international importance of associated sites, their susceptibility to adverse effects of environmental change, and the large aggregations of waterbirds of multiple species that they attract (( Kingsford and Auld 2005). Comparatively, far less is understood about habitat or site use of these species after dispersal or between breeding events (Caley et al 2022); yet critical population processes occur outside of breeding events with the potential to shape demography and population viability (Calvert et al 2009;Carrick 1962). For waterbirds in the Threskiornithidae family, it can take 3-4 years for juveniles to mature and recruit into the adult breeding population, during which time high rates of attrition occur (Robinson and McGinness 2024).…”

Identifying hypotheses for drivers of decline of the bogong moth (Agrotis infusa)

“…They can detect far away rainfall events, and may travel during good times to learn about the spatial layout of their environment, and use this to develop internal maps of wetland locations (Roshier 2009). This strategy seems to be working, with numbers estimated in the millions and little evidence they are undergoing longterm decline after correcting for seasonal conditions (Caley et al 2022). Compared to bogong moths, waterbird species have the advantage of much longer life spans, such that older birds may play an important role navigating to faraway resources (Reid 2009), with the mobility to utilise favourable conditions virtually anywhere in the eastern half of Australia.…”