| Abstract |
The birds are probably the only taxonomic group of animals that chooses only specific types of habitat for breeding, foraging and wintering. Habitat selection is based on structural characteristics of vegetation, foraging and nesting possibilties and presence of other species. The presence at the same habitat of morphologically similar species can be explained according to two hypotheses. The first supports the existence of competition of space and the second the differentiation of niches as a result of small morphological and physiological differents that leads to different ways of habitat exploitation. Both cases have been referred for several species of the genus Sylvia.
Apart from habitat selection another important part of birds’ life is their songs. They are long complicated sounds that are produced from male birds during the breeding period. Aim of the song is the defence of birds’ territories and the attraction of females. From sonograms (graphs of sound frequency in relation to time) of songs various parameters can be calculated, which can be used to compare song characteristics, not only among individuals of the same species but also among different species. An incident frequently occurring is the geographical variation of a species song and the existence of dialects, probably due to wrong learning of songs from young birds or due to different type of vegetation. Respectively because of habitat similarities, sympatric species can have song similarities. This can be also due to interspecific competition, because when a species sees a sympatric competitor to intrude to its territory area, it can imitate its song in order to defend it. Finally the songs of migrating birds are more complicated, because in this way males can attract easily more females, a fact that it is very important for their more restricted time available for breeding.
Among Sylvia species, S. melanocephala is a very common and widely distributed resident of Mediterranean basin, while S. rueppelli is a summer visitor with sporadic distribution in Eastern Mediterranean. Both species prefer similar breeding habitats and often occur sympatic in regions with maquis vegetation.
Aims of this project were to compare distributions and densities of S. melanocephala and S. rueppelli in common breeding areas, to study the habitat requirements of both species, to detect possible competition between them when they are sympatric, to check if there is geographical variation of their songs, to contrast a resident’s and a visitor’s song and to find similarities among songs of sympatric species.
Two areas of Southern Greece were choosen for study, Asterousia Mountain in Crete (seven stations) and Ymittos Mountain in Attica (one station), which have the typical maquis vegetation that both species prefer and it includes mostly shrubs of Q. coccifera, P. lentiscus and C. vilosa. During the period from 12 of April until 29 of May 2009 totally 117 samplings of number of pairs, vegetation characteristics and songs recordings were carried out. Distribution maps for Asterousia and Ymittos and densities histograms were drawn, while recordings were analyzed. Statistical analysis was done to check densities independence from vegetation characteristics and species and to compare song characteristics among regions for each species and also among species when they are sympatric.
The two highest densitiew occurs for S. melanocephala in two stations where at the same time S. rueppelli is absent. The third highest density occurs for S. rueppelli in one station from where S. melanocephala is absent. At the rest of the stations where the two species are sympatric, their densities are lower but almost similar. S. melanocephala in relation to S. rueppelli prefers lower altitude and slope, taller vegetation and denser shrubs. The most preferred shrubs for breeding are C. vilosa for S. melanocephala and Q. coccifera for S. rueppelli.
The resident S. melanocephala makes territories during the breeding period earlier than the summer visitor S. rueppelli, a fact that probably determines their later distribution. The species are either allopatric or sympatric, depending on the availability of space that exists in a region and on the space that S. melanocephala has used at the beginning. Highest densities are appearing when species are allopatric, while they are lower when species are sympatic. Densities are large because of the interspersed optimal habitats, which species perceive like “islands”. It seems that there is not obvious region fidelity of species for specific vegetation and from year to year. The differentiation of habitat selection between the species is a way to reduce their competition when they are sympatric.
Comparing songs there is not difference of song characteristics among regions for both species, meaning that there is no geographical variation of their songs. S. melanocephala has highest frequency with the highest peak, longer mean duration of song elements, shorter mean duration of pauses among song elements, fewer elements per song and smaller diversity of song elements. The song of S. rueppelli has more variation of element songs and it is more complicatied, as expected for migratory species that have less available time for breeding. Song characteristics similarities appear where the species are sympatric and it is mostly due to habitat similarities than due to species competition.
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