Evaluating spatiotemporal trends in terrestrial mammal abundance using data collected during bird surveys

Information on the status of biodiversity is crucial for species conservation and management. Large scale assessments are only feasible through citizen science but some taxa are poorly monitored because few people specialise in them. We explore alleviating this problem by using data collected for poorly monitored species as an add-on to existing bird surveys. Since 1995, participants in the annual Breeding Bird Survey have recorded the abundance of mammals during their surveys. We demonstrate the value of these data by developing spatial models of relative abundance for nine common and easily detected mammal species. Rabbit, brown hare and mountain hare all showed widespread declines. Conversely, deer showed increases throughout their ranges, with the exception of the red deer whose population was predominantly stable. The grey squirrel continues to increase in several areas. The red fox, the only carnivore with enough data, showed significant large declines. The collection of data on taxa other than the primary target has particular merit where the secondary taxa can be detected effectively by methods devised for the core survey. In such cases the data are inexpensive and inherit some of the benefits of the underlying structure and power of the core survey. However, the efficacy of the primary study design may vary for the members of secondary taxa and may not be temporally or spatially suitable for all of them. Although more volunteer training may be required, there are also opportunities to engage and enthuse people about conservation issues of other species groups.