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German L6pez-Iborra and Jose A. Gil-Delgado
ever, beech (Fagus sylvatica) forests do not fit the predicted pattern, since
they show low percentages of summer migrants (range 4.8-6.7%). Thus, it
appears that other factors besides the deciduous/evergreen dichotomy are
involved. One possibility is that the structural complexity of vegetation could
also play a role, and forests with more diversified vertical structure could
support a greater number of individuals of summer visitor species. This
could explain why beech forests, which usually lack a developed understory,
present so few summer migrant birds.
25.5 Dynamics of Selected Species
Table 25.1 shows the average density of the most abundant species in both
plots and the correlation coefficient between density in both plots for each
species. The seasonal pattern of variation for summer migrants is similar in
all species detected, since they arrive in May and remain in the forest until
August. However, the low densities that these species attain, especially in plot
2, prevent most correlations between their abundances in the study plots
from being significant (Table 25.1).
The seasonal dynamics of constant species are more varied. Although all
these species are present in the forest for most of the year, their response to
the changing conditions in the upper and lower parts of the catchment can
be different. This situation can be illustrated by some selected constant species (Fig. 25.4). The Robin and the Blackbird are species whose abundance is
similar in spring in both plots (Table 25.1), but that present greater density in
plot 1 in winter. With the Robin, the pattern of fluctuation is relatively similar
in both plots, with abundance maxima in summer and a significant correlation between plots (Table 25.1), although its density decreases in winter
much more in plot 1 than in plot 2. In contrast, the correlation between plots
is not significant for the Blackbird, indicating a different pattern of fluctuation in both plots. Blackbird abundance shows a summer maximum and a
winter minimum in plot 2, but in plot 1 it shows a relative maximum in
summer and then an absolute maximum in winter, in both years. Thus, it appears that in these two species, part of the population living at higher altitudes concentrates in winter at the bottom of the valleys, where the vegetation is more diverse and shrubs producing berries are more abundant. The
available data suggest that most of the Blackbirds remain in the forest in
winter, since its density in the central part of the winter in plot 1 is close to
the density in spring in both plots added together. On the contrary, a greater
proportion of Robins would leave the forest studied to live probably in manmade habitats at lower altitudes.
The tendency for bird density to be greater in plot 1 than in plot 2 during
winter is also observed in other species, such as the Blue Tit and the Chaffinch. These species presented similar abundance fluctuations in both plots
German L6pez-Iborra and Jose A. Gil-Delgado
ever, beech (Fagus sylvatica) forests do not fit the predicted pattern, since
they show low percentages of summer migrants (range 4.8-6.7%). Thus, it
appears that other factors besides the deciduous/evergreen dichotomy are
involved. One possibility is that the structural complexity of vegetation could
also play a role, and forests with more diversified vertical structure could
support a greater number of individuals of summer visitor species. This
could explain why beech forests, which usually lack a developed understory,
present so few summer migrant birds.
25.5 Dynamics of Selected Species
Table 25.1 shows the average density of the most abundant species in both
plots and the correlation coefficient between density in both plots for each
species. The seasonal pattern of variation for summer migrants is similar in
all species detected, since they arrive in May and remain in the forest until
August. However, the low densities that these species attain, especially in plot
2, prevent most correlations between their abundances in the study plots
from being significant (Table 25.1).
The seasonal dynamics of constant species are more varied. Although all
these species are present in the forest for most of the year, their response to
the changing conditions in the upper and lower parts of the catchment can
be different. This situation can be illustrated by some selected constant species (Fig. 25.4). The Robin and the Blackbird are species whose abundance is
similar in spring in both plots (Table 25.1), but that present greater density in
plot 1 in winter. With the Robin, the pattern of fluctuation is relatively similar
in both plots, with abundance maxima in summer and a significant correlation between plots (Table 25.1), although its density decreases in winter
much more in plot 1 than in plot 2. In contrast, the correlation between plots
is not significant for the Blackbird, indicating a different pattern of fluctuation in both plots. Blackbird abundance shows a summer maximum and a
winter minimum in plot 2, but in plot 1 it shows a relative maximum in
summer and then an absolute maximum in winter, in both years. Thus, it appears that in these two species, part of the population living at higher altitudes concentrates in winter at the bottom of the valleys, where the vegetation is more diverse and shrubs producing berries are more abundant. The
available data suggest that most of the Blackbirds remain in the forest in
winter, since its density in the central part of the winter in plot 1 is close to
the density in spring in both plots added together. On the contrary, a greater
proportion of Robins would leave the forest studied to live probably in manmade habitats at lower altitudes.
The tendency for bird density to be greater in plot 1 than in plot 2 during
winter is also observed in other species, such as the Blue Tit and the Chaffinch. These species presented similar abundance fluctuations in both plots