The Farmland Bird Indicator aims to give a general description of changes in bird populations. It is indeed possible that some bird species decline when the indicator is stable or goes up. Such a decline may be temporary due to natural fluctuations, but it may also be persistent, meaning that a species becomes endangered and deserves special attention. For this reason, PECBMS publishes not only multispecies indicators but also yearly supranational trends of individual species. As our indicator doesn´t capture all changes in biodiversity, which no single metric can, we also recommend adding a Red List indicator to summarize if the number of threatened species in a country increases or decreases over time (see Butchart et al., 2010).
Learn more in the related question, “If a species becomes extinct (or undetectable by the monitoring scheme because it is becoming scarce), does it also disappear from an indicator?“.