The intensive sampling will allow us to draw the abundance maps, to evaluate the ecological requirements, and to analyse the temporal changes. As stated before, this sampling will be founded on the SOCC program. Due to the importance of controlling the temporal changes during the study period, it is needed to give up part of the effort allocated to spatial representation to produce a fine temporal representation. Again, the SOCC program (whose censuses are repeated annually) should allow this temporal control. According to the current winter SOCC data, it seems that the interannual changes of the majority of landbirds will have an acceptable representation by means of this system. However, all the available data (from extensive and intensive surveys) will be needed for some species, and, therefore, the analysis of the interannual changes shown by those species poorly represented in the SOCC data will only be possible by considering groups of several UTM 10x10 squares as analytic units.

A preliminary assessment of the available resources and territorial requirements (number and location of transects) to obtain an accurate representation of the interannual changes leads us to consider that 160 squares would have to be surveyed by SOCC transects during the whole study period, the so called constant SOCC transects. The remaining UTM 10x10 squares will be distributed over the three years of field work. This subgroup does not include neither the boundary nor those coastal squares (21 total) that do not have space to hold a SOCC transect. Altogether, it involves up to 205 transects (Figure 8).

It is reasonable to use the SOCC priority transects as part of the 160 squares that need to be surveyed every year for the entire study period. Thus, 103 of the 160 constant squares will be covered by current priority SOCC transects; the remaining 57 will be selected among the rest of priority squares. Priority squares have been selected following a single criterion. Firstly, the territory has been subdivided into 5 main bioregions according to a cluster analysis from data belonging to the Breeding Bird Atlas, and, for every bioregion, a different number of priority squares have been selected (the larger the surface the bioregion occupies the smaller the proportion of squares selected): 8 for wetlands (100% of the squares), 12 for steppes (100% of the squares), 20 for high mountain (45% of the squares), 34 for mid mountain (40% of the squares) and 86 for Mediterranean areas (34% of the squares) (Figure 8). Within each layer (=bioregion), and once segregated from the analysis those squares that can not physically hold a SOCC transect, it has been done a random selection of the squares avoiding, when possible, taking adjacent squares, and, hence, maximizing the sampling spatial coverage. Considering the coverage of the project for the winter 2004-2005, the present coverage would be of 92 constant squares, with 68 still to be assigned. These figures are approximate, since some of the 92 squares hold SOCC transects of insufficient quality or that are located in habitats that poorly represent the priority environment in the square.

The 205 non constant SOCC transects, that is, those that will only need to be surveyed one year, follow the distribution of the extensive sampling. In this way, and in order to optimize resources, extensive and intensive surveys will be conducted for a selected square in the same winter. Thus, due to the restriction imposed by the order of the extensive sampling, the number of squares where SOCC have to be conducted for one single winter vary with the year: 57 in the first winter, 80 in the second and 63 in the last. We believe that this distribution of effort, peaking in the second year, is the one that fits best the evolution of square assignment in a project of this nature, according to the experience gained in the Breeding Bird Atlas. The present coverage of the non constant SOCC transects would rise up to 19 (33%) of the 57 to be conducted in the first winter, 22 (27%) of the 80 in the second and 22 (32%) of the 68 in the last.

The coordination of the intensive sampling will previously require locating potential transects inside each square. The key factor is obtaining an accurate representation, not only geographical but environmental as well. For this reason, it would be important controlling, as much as reasonable, the location of transects in the square. A problem would arise if the observers tended to systematically avoid certain environments (e.g. urban environments) and to concentrate in others. Due to the existing environmental diversity within a given UTM 10x10 square, each responsible for a square could propose the transect location; another solution could consist of pre-designs proposed by the coordination board whose suitability would have to be studied by the responsible for each square.



Figure 9. Distribution of the intensive sampling. 160 squares are to be surveyed each year (the colors vary according to habitat priority). They are distributed in such way that it will be possible to obtain an evenly distributed representation of the territory each year (see main text). The remaining squares (205) have been selected following the same criterion but imposing a restriction: if an extensive survey has to be conducted in a certain square in a certain winter, then an intensive survey will have to be conducted too.