364
German L6pez-Iborra and Jose A. Gil-Delgado
(Fig. 25.4; see also the high correlations between plots in Table 25.1), but
their density in winter was greater in the bottom of the catchment (Table
25.1), although the difference between plots was not as high as in the Robin
and Blackbird.
The abundance of other species did not differ between plots in winter.
This is the case with the Firecrest and the Great Tit (Fig. 25.4; Table 25.1).
The Firecrest presented similar abundance in both plots in both seasons,
with a tendency to be even more abundant in winter in plot 2 (paired t-test,
t = 1.99, df = 10, P = 0.07). This is the species with least pronounced seasonal
variations in abundance, being able to maintain relatively high densities in
winter in both plots. The abundance of the Great Tit decreased to low values
in winter in both plots, but in spring tended to be greater in plot 1.
25.6 Comparison of Seasonal Patterns of Density Fluctuation
in Iberian Forest Types
The seasonal pattern of fluctuation of total bird abundance in a community
should reflect the changes in resource availability throughout the year (Herrera 1980; Potti and Garrido 1986), which in turn will be affected by the climate and the specific composition of vegetation. The increase in the number
of studies on the composition of Iberian bird communities that present data
for a period of at least 12 months allows the comparative study of changes in
bird density throughout the year. In this way, we could determine whether
the pattern obtained in this study (summer maximum and winter minimum
of abundance), which is very often found in studies of temperate bird communities, can be extended to other forest types in the Iberian Peninsula, or if
other possibilities exist.
For this analysis, we transformed the total bird density obtained in each
month into a relative density with respect to the maximum density obtained
in each annual cycle. Then, the data for both years were averaged, thus obtaining a profIle of density variation for each plot. We compiled literature
data and analyzed them in the same way, thus transforming the absolute
density values of each month into relative values to the month of maximum
abundance, which received a value of 100. A matrix of similarity was obtained for the bird community data set using the Spearman rank correlation
index, and then this matrix was analyzed to construct a dendrogram
(Fig. 25.5) using the UPGMA algorithm.
This analysis shows that the simple pattern of summer maximum and
winter minimum of bird abundance cannot be generalized to all bird communities of Iberian forests. The dendrogram identified one group of bird
communities, including our study plots, which follows this pattern. It included not only holm oak woods, but also pine forests, deciduous oak forest,
and one non-forest habitat, a steppe in central Spain. The second identified
German L6pez-Iborra and Jose A. Gil-Delgado
(Fig. 25.4; see also the high correlations between plots in Table 25.1), but
their density in winter was greater in the bottom of the catchment (Table
25.1), although the difference between plots was not as high as in the Robin
and Blackbird.
The abundance of other species did not differ between plots in winter.
This is the case with the Firecrest and the Great Tit (Fig. 25.4; Table 25.1).
The Firecrest presented similar abundance in both plots in both seasons,
with a tendency to be even more abundant in winter in plot 2 (paired t-test,
t = 1.99, df = 10, P = 0.07). This is the species with least pronounced seasonal
variations in abundance, being able to maintain relatively high densities in
winter in both plots. The abundance of the Great Tit decreased to low values
in winter in both plots, but in spring tended to be greater in plot 1.
25.6 Comparison of Seasonal Patterns of Density Fluctuation
in Iberian Forest Types
The seasonal pattern of fluctuation of total bird abundance in a community
should reflect the changes in resource availability throughout the year (Herrera 1980; Potti and Garrido 1986), which in turn will be affected by the climate and the specific composition of vegetation. The increase in the number
of studies on the composition of Iberian bird communities that present data
for a period of at least 12 months allows the comparative study of changes in
bird density throughout the year. In this way, we could determine whether
the pattern obtained in this study (summer maximum and winter minimum
of abundance), which is very often found in studies of temperate bird communities, can be extended to other forest types in the Iberian Peninsula, or if
other possibilities exist.
For this analysis, we transformed the total bird density obtained in each
month into a relative density with respect to the maximum density obtained
in each annual cycle. Then, the data for both years were averaged, thus obtaining a profIle of density variation for each plot. We compiled literature
data and analyzed them in the same way, thus transforming the absolute
density values of each month into relative values to the month of maximum
abundance, which received a value of 100. A matrix of similarity was obtained for the bird community data set using the Spearman rank correlation
index, and then this matrix was analyzed to construct a dendrogram
(Fig. 25.5) using the UPGMA algorithm.
This analysis shows that the simple pattern of summer maximum and
winter minimum of bird abundance cannot be generalized to all bird communities of Iberian forests. The dendrogram identified one group of bird
communities, including our study plots, which follows this pattern. It included not only holm oak woods, but also pine forests, deciduous oak forest,
and one non-forest habitat, a steppe in central Spain. The second identified