Fig. 4
From: Estimating the movements of terrestrial animal populations using broad-scale occurrence data
A worked example exploring observation trends in eBird occurrence data from 2008 to 2019 for two closely related species: migratory Black-chinned Hummingbird (Archilochus alexandri) and range-expanding Anna’s Hummingbird (Calypte anna) [125] from the western flyway of North America [36]. Even closely related species can display different dynamics, which can dramatically affect how the data is structured across space and time. (A) The total number of checklists (log10 transformed) containing each species increases through time, which is expected as the crowdsourced platform gains new observers. It does not represent an increase in the total number of hummingbirds. (B) In contrast, the percent of all checklists containing each species within regions where each occurs is declining for Black-chinned Hummingbirds, and increasing for Anna’s Hummingbirds, which may reflect changes in observer behavior, expertise, or geographic coverage through time. (C) After spatially binning the data, the number of unique grid cells in which each species is observed increases slightly through time, but is relatively flat in recent years, giving some confidence that the species’ locations have been adequately covered through the time frame and within the spatial area. (D) The number of days that the species was observed in each year is flat for Anna’s Hummingbird after 2008, indicating that they were observed every day in each subsequent year. In contrast, Black-chinned Hummingbirds show a strong increasing trend, which suggests a need to further explore the data to see whether it indicates increased observer effort in general, or at particular locations or times of the year, or if it represents a meaningful ecological trend in the occurrence phenology of the species