Influência do bico na estrutura do canto em papa-capins neotropicais (Thraupidae: sporophila)

Abstract

Acoustical signals are the main form in which birds communicate, and they are used to attract mates, defend territories, alarm others from predators and recognize each other, being than an important feature on speciation process. Then, song is extremely important, especially in Oscines, a group of birds that have a widely types of songs, singular for each species. Many studies have been shown that the vocal tract act as a constrict factor on song structure, besides body size. Beak gape, size and bite force are known as a limiting factors of note rate, frequency bandwidth and maximum/minimum frequencies, in which larger beaks produces narrow sounds with lower note rates. Our study had two objectives related of Oscine birds (of the genus Sporophila): the first one (chapter I) was to analyze the evolution of beak volume and song structure and verify the existence of song constriction and coevolution between them. The other objective (discussed in chapter II) was to test the relation between beak volumes and bite force with the song structure of three sympatric species of Sporophila, and try to infer how they organize their songs to avoid the acoustic overlap due their coexistence. In the chapter I we analyze territorial songs of 47 males from 11 species of Sporophila, representatives of Neotropical Oscines (Thraupidae). We found that frequency bandwidth and maximum frequency had a negative relation with beak volume, besides that, minimum frequency showed a trend for the same pattern. Beak volume, note rate and minimum frequency demonstrated to have a phylogenetic signal, which suggest that closely related species had beak morphology aand song structure more similar then lesser related species. Our results corroborate literature, in which beak volume acts as a limiter for song structure and also that this character can coevolve with birdsong. However, other variables did not present a constriction by the beak volume, what suggest that there is much to discover about this relation in songbirds yet. It is interesting to test the phylogenetic influence besides morphological characters, since this feature can be influencing song structure more than song constrainers. In chapter II, we selected S. beltoni, S. caerulescens and S. hypoxantha as a model of large, medium and small species respectively. These three species used to occur in the same habitat where they breed in northern of Rio Grande do Sul, Brazil. Adult that were observed and recorded were captured, had they beak measured and pass though bite force test. We found that S. beltoni and S. hypoxantha had more differences among song parameters, in which the first one presents the larger beak among the three species evaluated, and the second the smallest beak. The highest relation between strength/beak size and song was observed in S. caerulescens in which frequency bandwidth and minimum/maximum frequencies presented a positive relationship with bite force and beak size, showing that this feature do not limit song on this taxa against that other studies have shown. Concerning to the use of acoustical space, the greatest differences found were related with note rate and song duration. These factors should be some features used to avoid the song overlap on nature. In general terms our study shows that the limitation of sound per characters of the beak it is not a rule in Oscines, although there is a negative relationship between song variation and beak size when more species are add to the model. With these results we belive that it is important to know how changes in morphology of all vocal tract can affect song structure and how morphologic and acoustic features had evolved through their phylogenetic history, because understand this is essential to better comprise the speciation process among these groups.