Using drones and citizen science counts to track colonial waterbird breeding, an indicator for ecosystem health on the Chobe River, Botswana

Abstract

Many of the world’s freshwater ecosystems are degrading with increasing water resource developments, necessitating identification of useful indicators that track large scale environmental change. We investigated the effectiveness of waterbird breeding at the Kasane breeding colony as a biotic indicator of ecosystem health related to river flow and flooding on the Chobe River in Botswana. We counted nests and birds in the breeding colony in 2018 and 2019 using drone surveys, and estimated three breeding success indices: reproductive success, nesting success and fledgling success. We estimated 4529 birds of six species (African darters Anhinga rufa, African openbills Anastomus lamelligerus, African spoonbills Platalea alba, reed cormorants Microcarbo africanus, white-breasted cormorants Phalocrocorax lucidus and yellow-billed storks Mycteria ibis) were present with 2861 nests in early August 2018; there were no nests counted in 2019. Reproductive success differed significantly among species, ranging from 81% of eggs fledged for yellow-billed storks to 47% for African openbills and was significantly related to Chobe River level. To explore historical waterbird presence in the colony, we also investigated relationships between citizen science waterbird counts of the six breeding species on the river (1993–2018), to intra and inter-annual variation in river flows, flooding and local rainfall. Waterbirds were significantly less abundant at times of higher inundation throughout the year, and inter-annual maximum species abundances, likely indicative of breeding, were positively related to Chobe River maximum levels, occurring when the annual maximum Chobe River level was > 4.33 m. This matched our data: breeding in 2018, when the river level reached 5.09 m, and no breeding in 2019, when the river level was 3.23 m. Breeding of colonial waterbirds can be a useful indicator for ecosystem health as it requires sufficient and varied food sources at large scales, often in response to extensive inundation. It can be tracked using drone surveys, a non-invasive and safe way of estimating reproductive success. The determination of historical and current relationships between waterbird breeding and river level and flow can be used to forecast effects of climate change and water resource development on aquatic ecosystems.