Yes, they all show exactly the same set of squares for each species. They just differ in the type of information provided, occurrence, breeding evidence and abundance, respectively.

50-km occurrence maps simply show the presence or absence of the species in a given square according to field observations either in the EBBA2 period (mainly 2013–17) or EBBA1 period (mainly 1980s). Presence data refer to either possible, probable or confirmed breeders (see next question).

Birds are mobile species and can be observed far from the areas where they reproduce, e.g. as visitors during migration or post-breeding dispersal. 50-km breeding evidence maps show the maximum observed likelihood that the species breeds in every square. Three standardised categories are used to determine this likelihood: possible, probable or confirmed breeder. These categories are based on the type of observation done in the field, which is standardised in 16 atlas codes.

Abundance is a key parameter to complement the distribution of bird occurrence and breeding likelihood, and as such is widely used in research and conservation. Abundance maps show the information on the number of breeding pairs per square reported by national coordinators. Depending on the characteristics of the species and the available data, three main approaches were essentially used to collect these data, namely direct counts, statistical inference and expert assessment. The first was mainly used for scarce but conspicuous species for which counts were available, the second was often used for widespread and common species, and the third if other methods were not possible.

Breeding evidence maps show the highest category reported by any of the countries. For abundance the information from the countries concerned was pooled.

Modelled maps show patterns of probability of breeding occurrence in Europe at 10-km resolution. These maps should be mainly interpreted at European scale and treated with caution at more restricted spatial levels, where other, local sources of information are likely to be more reliable. Despite these limitations, EBBA2 modelled maps very likely represent the best maps of breeding occurrence ever developed for Europe as a whole, and also for many parts of the continent where information at this level of detail has not been available so far.

Modelled maps show relative rather than absolute values of probability, so values can be compared within a single map but not necessarily between modelled maps of different species. Patterns of occurrence are better compared between species by means of the 50-km abundance maps (although the details of the modelled map are lost).

Abundance maps show estimations of the number of breeding pairs per 50-km square, and modelled maps show probability of occurrence per 10-km square. For many species, in particular widespread passerines, modelled maps can be interpreted as gradients of relative abundance. This is why the geographical patterns of abundance maps and modelled maps are often similar. However, some caution is required here, because probability of occurrence may remain constant above certain thresholds of abundance. On the other hand, modelled maps may not reflect abundance well, e.g. for species with a clumped occurrence. Finally, it should be taken into account that modelled maps were generated for the whole of Europe at once, while values in the abundance maps were provided country by country.

No. EBBA2 change maps only show changes in the species presence between the two atlases. They basically show squares where the species was found in one or the other atlas, or in both.

Overlaying the maps of EBBA1 and EBBA2 can give you a rough idea about change but we do not advise you to make these comparisons, because important information regarding the quality of the data will be lacking. Please use the change maps instead, for which the data have been controlled for effort in data collection.

Change maps are restricted to the continuous geographical area that was well covered in both atlas periods. Since the coverage was far more complete in EBBA2 than in EBBA1, this means that the area analysed in change maps includes areas homogeneously covered in EBBA1. Thus, Cyprus, the Asian part of Turkey, the Canary Islands, most of the European parts of Russia and Kazakhstan, Georgia, Armenia and Azerbaijan were excluded from the change maps.

Not exactly. Although the basis of the change map are the data from these two atlases, the squares used for the comparison and the calculation of the change index are a subset of the total number of surveyed squares. The set of squares selected for its inclusion in the change maps are those that we consider that better approach the real changes on species occurrence within the 30-year study period. Information on completeness of coverage, species richness plus a few less important criteria were used to determine the set of squares qualifying for robust comparison between the two atlases. We recommend the use of the data underlying the change maps for further analyses of change in species occurrence. See further information in the Methodology and the book.

Change maps give an impression of the spatial pattern of gains and losses in the distribution of a species but do not show the magnitude of change. We quantified the change in distribution by calculating a change index as: Change index = 100 * (N2–N1)/(N2+N1), where N1 and N2 are the number of occupied squares for a given species in the EBBA1 and EBBA2 periods, respectively. Positive and negative values of the change index indicate the extent to which distribution has increased or decreased: 0 indicates no change, –100 extinction and +100 colonisation. This index has the advantage of keeping increases and decreases symmetrical in magnitude, which facilitates comparisons between species. However, the index should not be interpreted as a percentage change from a past reference. See further information in the Methodology and the book.

Not a straightforward manner. At large scales such as Europe, distribution expansions are often related (actually favoured) to population increases. And the opposite is also often true, i.e. distribution shrinking is linked to an overall population loss. However, this is not always the case. In some cases distribution and population size can change in a divergent manner. For example, this happens when a species becomes concentrated in particular areas in which habitat quality has increased over time. For changes in population size please consult trends reported by the PECBMS project and European Red List of Birds.

Atlas work is not an easy task and it takes years (10 in the case of EBBA2) to accomplish such a huge project. A total of 30 years have passed between the EBBA1 and EBBA2 fieldwork periods. However, the EBCC is fully conscious of the interest of potential updates in the near future.