Fieldwork method

The surface of the National Park in its Madrid sector was divided into 330 squares of 1×1 km to distribute sampling across the entire area using a standardized and easily georeferenced sampling unit (Figure 1).

Sampling in each unit was carried out through transect walks that aimed to cover all habitat types and therefore detect all species present. During these surveys, all birds seen or heard were recorded so that an inventory could be obtained for each unit, allowing the distribution map of each species and the 1×1 km bird inventory to be determined.

Figure 1. Study area and boundary of the Sierra de Guadarrama National Park in its sector within the province of Madrid. Grid of 330 UTM squares of 1×1 km overlaid on the map area.

Fieldwork was carried out more intensively during the core breeding season in order to detect the majority of species (15 April – 15 June), although visits were also conducted outside this period to identify species that breed earlier (large raptors) or later, such as some trans-Saharan migrants with late breeding behaviour.

The basic approach in each grid square consisted of performing a standardised and common survey route for all units, aimed at establishing relative abundance by counting the number of individuals detected within a band. The route was carried out, whenever possible, within a specific habitat type (quantitative work). In addition, all patches of other habitat types were visited to detect additional species present within the unit. The basic characteristics of the quantitative and qualitative surveys were:

Quantitative. In each 1×1 km UTM grid square, a 500-m transect was performed, recording the number of contacts for each individual detected and the main habitat.

• Each transect was established, whenever possible, to run through homogeneous environments.

• All detected species (seen or heard) and the number of individuals of each were recorded, distinguishing those within a 25-m band and those outside it.

• Surveys were conducted from dawn until mid-morning (10–11 a.m.), dedicating the rest of the time allotted to the unit to qualitative work (identification of species not detected during quantitative transects).

• All data were recorded using SEO/BirdLife’s mobile application “Atlas y Muestreos”, digitally storing bird and habitat information immediately, as well as all associated metadata, tracks, dates, times, etc.

 

This information made it possible to determine species abundance and richness per grid square and habitat type.

The habitat types recorded were classified according to the framework presented in Annex 1.

In parallel, additional time was allocated within each sampling unit to visit other habitat types present in the square and identify the remaining species that could be present.

Qualitative. Once the quantitative transects were completed, additional areas within each grid square were visited to detect as many species as possible that had not been recorded during the quantitative work (500-m transect in each 1×1 km UTM square; Figure 2), as well as evidence of breeding categories. The central hours of the day (when bird activity is lower) were avoided, and the late afternoon hours were used.

• The qualitative survey consisted of walking through as many areas as possible within the grid square in order to detect the highest possible number of different species and their breeding activity, noting the corresponding breeding code for each observation (Table 1).

• All data were recorded using the mobile application “Atlas y Muestreos”, digitally storing bird and habitat information immediately, as well as all associated metadata, tracks, dates, times, etc.

 

Figure 2. Example of surveys carried out in each 1×1 km UTM grid square. The quantitative transect is shown in red and the qualitative routes in green.

 

0. NON-BREEDING OR PRESUMABLY NON-BREEDING (Does not breed in the grid square or is suspected to be migrating, vagrant, feeding in the area, etc.)

A. POSSIBLE BREEDING

1. Species observed during the breeding season and in possible breeding habitat

2. Singing male or present (territorial song heard) during the breeding season

B. PROBABLE BREEDING

3. Pair observed during appropriate breeding season and in suitable habitat

4. Species observed showing apparent territorial behaviour on two or more different days within the same week in the same location

5. Courtship behaviour

6. Visiting a possible nest

7. Adult showing mating behaviour or mating calls

8. Incubation patch observed on an adult in hand

9. Nest building or excavation of holes

C. CONFIRMED BREEDING

10. Distraction display or defence of territory

11. Used nest or eggshell remains

12. Recently fledged young (nidicolous) or very small chicks (nidifugous)

13. Adults near nests or holes indicating occupation or incubation

14. Adult carrying faecal sac or food for chicks

15. Nest with eggs

16. Nest with chicks seen or heard

Table 1. Breeding categories considered.

Annex 1. Types of habitats considered.