Territory occupation sequence and population change 2005–2019 in a satellite versus core area for Grasshopper Warblers Locustella naevia

Because of habitat preferences and variation at a landscape level, a species’ distribution tends to show a level of aggregation. In the Grasshopper Warbler Locustella naevia, the distribution is linked to suitable breeding habitats of open grassy and herbaceous grounds, often adjacent to water. Consequently, the presence of the Grasshopper Warbler at landscape level will consist of core and satellite areas. In theory, birds can sense habitat quality and should occupy territories within those areas based on territory quality. It might also result in different population trends between different areas in the landscape. I tested these assumptions through a 15-year study in a satellite area, comparing the results to a nearby core area. In both areas, the males occupy a territory based on the perceived attractiveness, and general patterns of the territory utilization were similar between areas. Territory density was lower and the males arrived later in the satellite area, thereby confirming the satellite/core area relationship between the study areas. In the core area, no significant change in population size was noted, while in the satellite area, the population decreased significantly.