Effects of density‐dependence and weather on population changes of English passerines using a non‐experimental paradigm

The Common Birds Census documents changes in the populations of the more abundant British land birds. Here we analyse the CBC data for various English passerines to discover the separate effects of weather and of density‐dependent feedback on their annual population changes. Density dependence is generally apparent in the data from woodland plots, less so in those from farmland. There are clear effects of weather, particularly in farmland. Prolonged snowfall in winter reduces populations of most species; frost and low temperatures appear much less important. Rainfall in March and April increases numbers censused in the spring but this may be an artefact. We discuss ways in which analyses such as these should be taken forward. We consider whether this work is respectable science, arguing that monitoring through the CBC goes much further than mere surveillance of numbers, that such monitoring is important in wildlife management, and that density dependence is not a bankrupt paradigm. Long‐term data gathering is an essential part of ecological science, even in programmes not designed at the outset to test specific hypotheses.