Questions and answers

Compiled by Petr Voříšek, Arco Van Strien, Willy Van Strien, Jana Škorpilová, Ian Burfield and Richard D. Gregory

Updated on 11 November 2017

In a form of questions and answers, this section aims to help understanding methods of data collation and analysis within the Pan-European Common Bird Monitoring Scheme. The section is complementary to the section Methods. We expect the Questions and Answers will be updated on regular basis and appreciate suggestions for improvements or completely new questions on methods of production and interpretation of supranational species population trends and indicators. Please send comments and suggestions to Petr Voříšek, the PECBMS manager,

1. National data sources, field work

  • 1.1 Data are collected mainly by volunteers. How it is assured that volunteers properly collect the data? Do volunteer based monitoring schemes produce results that are as trustworthy as those from professional fieldworkers would be? Answer.
  • 1.2 There is a turnover of volunteer observers in every scheme. Some volunteers stop after some time and others enter the scheme. Doesn´t this turnover affect the results, given that volunteers have different observational skills? Answer.
  • 1.3 Sampling the same sites every year is fine for farmland that is maintained, but in forests the species composition will change in the course of 25 years due to succession. Do sampling sites have to be replaced if they change strongly? Answer.
  • 1.4 It is impossible to count all birds present at a site during a visit, as not all birds are observed. How does this affect the results? Answer.
  • 1.5 There is some evidence that climate change causes shifts in the phenology of species. Is it possible that a detected decline in numbers of a species is caused by the fact that the observers missed birds because these birds nowadays are singing or nesting a few weeks earlier than in former years? Answer.

2. National species trends and indices (some questions apply to the supranational level too)

  • 2.1 What is TRIM, how does it work, where can it be obtained? Answer.
  • 2.2 What is BirdStats, how does it work, where can it be obtained? Answer.
  • 2.3 What exactly do year to year changes in species index values stand for? Answer.
  • 2.4 Is it possible that plots have been sampled in a non-random fashion at the start of the national monitoring schemes – because observers tended to count at sites with a high species richness and abundance of birds – biasing the results for the first years? Answer.
  • 2.5 Do we have to worry about each local decline of a species? For this may be compensated by increases elsewhere, resulting in a stable species trend. Answer.
  • 2.6 When TRIM is updated and improved, is it necessary to recalculate all time series? Answer.
  • 2.7 Is it necessary to recalculate all time series when new data become available each year? Answer.
  • 2.8 Are missing values and the way they are imputed in PECBMS typical for bird monitoring? Answer.

3. Supranational species trends and indices (some questions apply to multispecies indicators too)

  • 3.1 Does PECBMS check the quality of the national data collected, and how? Answer.
  • 3. 2 The supra-national species indices and the multispecies indicators are based on adding national results. Is this justified, as the borders between countries are artificial?  Answer.
  • 3.3 National indices are combined to supranational ones. To this end, missing yearly indices of a particular country (national all-sites totals) are imputed from data of countries within the same region (West, North, East/Central, South, South East). Is it justified to do this? Can we e.g. really expect that data from France can be used to estimate missing values in Spain? Answer.
  • 3.4 National indices are weighted when combined according to national population size estimates (BirdLife International, 2004), but the quality of population estimates is very different between countries. Does it mean that weights are based on incorrect values? Answer.
  • 3.5 Instead of national population size estimates, other weights may be used in the calculation of supra-national population indices per species, e.g. area of habitat or precision of trend estimates. Why does PECBMS only apply population size estimates? Answer.
  • 3.6 Is it possible that early years results are biased towards western Europe, as monitoring schemes started later in many other countries? Answer.

4. Multispecies indicators: species selection and classification

  • 4.1 What species habitat classification is used for PECBMS data? Answer.
  • 4.2 Why is a multispecies indicator better than one based on a few species or even a single species? How many species should be included in an indicator? Should we aim for as many species as possible, or for the opposite? Answer.
  • 4.3 If a species becomes extinct (or undetectable by the monitoring scheme because it is becoming very rare), does it also disappear from an indicator? Answer.
  • 4.4 Which criteria are used to include a particular species in an indicator or not? E.g. why is Picoides tridactylus not included in the forest bird indicator? Answer.
  • 4.5 How did PECBMS classify the species that are characteristic for each habitat type? Has there been any change in this procedure since the start? Answer.
  • 4.6 Is the classification of species to be included in the (farmland) bird indicator similar across countries? Answer.
  • 4.7 National farmland bird indicators presented on the Eurostat web site are based on the European species classification developed and used by PECBMS. Why aren´t they based on the existent national classification systems? Answer.
  • 4.8 Many species occur in more than one habitat and their trends can be different in those different habitats. To produce indicators for a particular habitat type, PECBMS includes the species that were classified as characteristic for that habitat and then uses all count data for those species. Wouldn´t it be better to use only the data from the habitat type under concern? Answer.

5. Multispecies indicators: production

  • 5.1 In producing multispecies indicators, all species are weighted equally. Wouldn´t it be better to weigh them differently, for instance to give specialists more weight? Answer.
  • 5.2 Why are indicators not presented at a logarithmic scale in the graphs? Answer.
  • 5.3 How sensitive are the indicators to changes in species selection? Answer.
  • 5.4 How sensitive is the indicator to population changes in one or a few species? Answer.
  • 5.5 Some countries produce their own bird indicators. Are these indicators similar to the PECBMS indicators? Answer.
  • 5.6 Why do the national farmland bird indicators of PECBMS differ from the national farmland bird indicators used in my country? Answer.

6. Multispecies indicators: interpretation

  • 6.1 The indicator values in early years are different between indicator updates. Does it mean that the indicator is not robust and credible? Answer.
  • 6.2 What exactly does the multispecies bird indicator tell us about changes in bird diversity? If the indicator goes down, is bird diversity deteriorating? If the indicator goes up, is bird diversity improving? Answer.
  • 6.3 Isn´t the Farmland Bird Indicator incomplete or even deceptive, as it may go unnoticed that certain species decline and may run the risk to become extinct? How can we solve this problem? Answer.
  • 6.4 Is there any evidence that birds are good indicators for biodiversity as a whole? Do population trends of birds reflect trends in other taxa? Answer.
  • 6.5 What are the likely causes of the decline in the farmland bird indicator? Answer.
  • 6.6 What are the likely causes of changes in the forest bird indicator? Answer.
  • 6.7 What is the evidence that the changes reported in farmland birds are due to agricultural change? Answer.
  • 6.8 There is a decline in the farmland bird indicator and this is attributed to agricultural intensification. However, we know that other drivers play their role, for example climate change. For instance, Lapwing Vanellus vanellus, part of the farmland bird indicator, is supposed to decline as a result of climate change. Increasing numbers of predators and invasive species might also be important. How can we be sure that declining trends in farmland birds are driven really by changes in farmland and not by other forces? Answer.
  • 6.9 Is extreme weather taken into account as a possible explanation of decline of farmland and forest birds? Answer.
  • 6.10 Can the farmland bird index tell me about the effectiveness of specific agri-environmental measures? Answer.
  • 6.11 The farmland bird index is rather stable in recent years, thus the target (to halt decline of biodiversity) has been achieved. Answer.