Forum es una sección que pretende servir para la publicación de trabajos de temática, contenido o formato diferentes a los de los artículos y notas breves que se publican en Ardeola. Su principal objetivo es facilitar la discusión y la crítica constructiva sobre trabajos o temas de investigación publicados en Ardeola u otras revistas, así como estimular la presentación de ideas nuevas y revisiones sobre temas ornitológicos actuales.

The Forum section of Ardeola publishes papers whose main topic, contents and/or format differ from the normal articles and short notes published by the journal. Its main aim is to serve as a lighter channel for discussion and constructive criticism on papers or reseach lines published either in Ardeola or elsewhere, as well as to stimulate the publication of new ideas and short revisions on current ornithological topics.

Between 1990 and 2008 three independent estimates of national population sizes for common breeding birds have been produced in Spain. These estimates have been generated using three different methods: extrapolation of published information on bird densities; a rough assessment of population size using data collected by volunteers during breeding bird Atlas fieldwork; and density values obtained from a bird monitoring scheme based on point-count stations. In addition, an estimate of population size for common breeding birds has been produced in Catalonia, a Spanish autonomous community, using data from a bird monitoring programme based on line transects. The estimated relative abundance of bird species was similar for all these methods but absolute values differed up to 30-fold. The differences between the two more comprehensive datasets showed that the most recent national estimates were on average nearly five times as high as earlier ones, and more than ten times greater for 19% of species. The differences between estimates were especially notable for species associated with urban habitats. Although the estimates span a period of about 20 years, change in bird populations across time was not the main factor driving the observed discrepancies. Differences in sampling design and data analysis were probably responsible. Particularly important factors that account for these differences would be: (i) the absence of sampling design and the limited availability of data in earlier estimates; (ii) the more recent sampling of habitats, notably urban environments and farmland, which previously had not been well surveyed, thus increasing the population size estimates of species living there; (iii) biases resulting from the choice of sampling sites by observers, (iv) the use of different methods to estimate population density. To improve future population estimates it is suggested that additional work is needed to understand the shortcomings of bird monitoring programmes that are the data source for population size estimates of common birds. The results should also be disseminated in peer-reviewed journals. Ultimately, a refinement of the existing monitoring schemes or even the implementation of new approaches may be necessary to obtain reliable estimates of the population sizes of common birds.