The role of dispersal in range change in birds

Eurasian reed warbler Acrocephalus scirpaceus expanded its range in Great Britain in the late 20th and early 21st centuries. The role of dispersal in this range expansion was investigated. Inference of the mechanisms underlying the range dynamics drew on fieldwork, analysis of large observational datasets, and a simulation model; this model was run in a reedbed map of Britain, generated from satellite data using machine learning. Breeding season temperature sets up reed warbler’s range limit in Britain directly, by influencing occupancy in the current year, perhaps mediated through reed Phragmites australis phenology. Although components of productivity were positively related to temperature, these and adult survival did not decline to the range edge. There was therefore no evidence that demography plays a role in limiting reed warbler’s range in Britain; however, not all aspects of demography were investigated. Survival was negatively related to temperature, and simulations suggested that this may allow reed warbler to maintain a more northerly range limit than without such a relationship. Reed warbler’s range expansion can be explained by a gradual equilibration with climate space, enabled by long-distance dispersal: only rare long-distance dispersing individuals matched the rate of range expansion. Reed warbler’s range edge tracked climate change, but the bulk of the population lagged behind. This could be due to dispersal-limitation, or perhaps newly established populations grow too slowly to generate sufficient emigrants. Simulations suggested that reed warbler’s range size is more sensitive to demography than to dispersal. The number of fledglings per breeding attempt increased over time, probably due to climate warming, and could have increased emigration; if so, this may be the cause of a more rapid movement in the range centroid later in the study period. Emigration, transition and immigration may therefore play different roles in reed warbler’s range dynamics in space and time.