The sampling system, designed to obtain high-resolution density maps, population estimates and bird-environment relationships, differs considerably from that used for the Catalan Breeding Bird Atlas 1999-2002. Indeed, apart from methodological considerations that will be mentioned, it is recommendable to adapt as much as possible this new atlas to the present and well established Catalan monitoring program (Catalan Common Bird Survey), to which more than 160 ornithologists contribute. Otherwise, shortage of human resources to keep independently running both projects could arise. The objective of obtaining more precise quantitative maps also recommends the use of a lineal transect-based data collection system, which also yields relative densities, not only presence-absence data.
The winter SOCC also consists of a 3-km transect divided in six 500-meter sections along which all the individuals, either seen or heard, are counted and recorded by section. Two censuses are to be conducted: one in December and one in January. These should preferably be done in the first 15 days of each period, and there should be at least 15 days apart. All censuses have to be carried out in the morning, between sunrise and the next four hours. Timetables can vary according to the local conditions; however, it should be taken into account that the pilot tests conducted in the 2005-2006 winter show that the censuses can not be carried out in the afternoon (more information: intensive method, census timetable; PDF file, 313Kb). There are two SOCC variants: standard (without bands) or expanded (with three bands to each side of the observer: 0-25, 25-100 and >100 m). The expanded SOCC allows calculating the detectability coefficient for every species, and through it obtaining absolute densities in birds/km2. For this reason, collaborators will be encouraged to adopt the expanded SOCC.
206 transects were conducted during the winter 2002-2004, whereas 3.077 UTM 1x1 squares were surveyed during the four years of field work of the Breeding Bird Atlas. The division of each transect into three 1-km sections and the application of the same statistical models employed for the Breeding Bird Atlas has yielded very promising results (Figures 1 and 2) for the breeding season, since those for the 1x1 squares agree with those for the spring SOCC. Some models still require a higher coverage in some areas and more data from certain environments (Figures 3 and 4), but it is likely that a guided increase in the current SOCC coverage within the new project framework, will help fine-tuning the results. The first winter distribution maps also seem very promising too (Figure 5) but show local weaknesses because of a lack of coverage (Figure 6).
Another challenge, just beginning to be thoroughly explored by means of the SOCC data, is mapping the interannual differences in distribution. The first trials conducted are based on the interpolation of the results given by the TRIM (statistical package used for the assessment of annual trends and abundance indexes). The current perception is that ecological niche-based models are suitable to map average distribution over a number of years (Figures 1-6), but it seems that, at a nationwide scale and with the available variables, interpolation is conceptually more appropriate to reflect the relation between interannual change and environment.
After these preliminary trials, it seems adequate to use the SOCC program to draw high-resolution maps. Moreover, there are other reasons to believe it is a correct decision, since it is an ongoing project that is yielding of data and that mobilizes many collaborators during the winter. On one hand, the integration of the results obtained through the diverse ICO projects is a goal in itself. This approach was already initiated with the Breeding Bird Atlas, which used data from the SOCC and SYLVIA programs. Integrating from the beginning these projects may generate very positive synergies for all them. On the other hand, within the framework of the European Bird Census Council (EBCC), the ICO and the CTFC (Technological Forestry Center of Catalonia) are actively promoting the elaboration of pan-European high-resolution maps based on common bird monitoring programs. It seems reasonable to apply at home those technical proposals that we support abroad.
Figure 1. Above: Map of abundance of the Blue Rock Thrush Monticola solitarius drawn after data from the spring SOCC transects (average of years 2002-2004, units in ind/km). Below: Abundance index map published in the Catalan Breeding Bird Atlas 1999-2002 (the unit is the probability of appearance).
Figure 2. Above: Abundance map of the Sardinian Warbler Sylvia melanocephala drawn after data from the spring SOCC transects (average of years 2002-2004, units in ind/km). Below: Abundance index map published in the Catalan Breeding Bird Atlas 1999-2002 (the unit is the probability of appearance).
Figure 3. Above: Abundance map of the Corn Bunting Melanocorypha calandra drawn after data from the spring SOCC transects (average of years 2002-2004, units in ind/km). Below: Abundance index map published in the Catalan Breeding Bird Atlas 1999-2002 (the unit is the probability of appearance).
Figure 4. Above: Abundance map of the Greenfinch Carduelis chloris drawn after data from the spring SOCC transects (average of years 2002-2004, units in ind/km). Below: Abundance index map published in the Catalan Breeding Bird Atlas 1999-2002 (the unit is the probability of appearance).
Figure 5. Above: Abundance map of the Black Redstart Phoenicurus ochruros drawn after data from the winter SOCC transects (average of years 2002-2004, units in ind/km). Below: Abundance index map published in the Catalan Breeding Bird Atlas 1999-2002 (the unit is the probability of appearance).
Figure 6. Above: Abundance map of the Song Thrush Turdus philomelos drawn after data from the winter SOCC transects (average of years 2002-2004, units in ind/km). Below: Abundance index map published in the Catalan Breeding Bird Atlas 1999-2002 (the unit is the probability of appearance).
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