Browsing by Author "Välkki, Anna"

Birdsong is information, communication and a target of sexual selection. Song complexity is the variation in bird song structure. Within-song complexity is variation in song components, in number and order of syllables and phrases. Song complexity is an honest trait under sexual selection and competition for mates increases it. When species expands its range front to a new area, the population density is low in the beginning. Males in the new population do not need to compete as hard as before, because population density is decreased, which decreases song complexity. Many warblers have complex songs and they do not have a colourful plumage. It is well possible that song complexity is the main target of sexual selection in warblers. The purpose of this study was to investigate if there were differences in song complexity in Eurasian reed warblers (Acrocephalus scirpaceus) breeding at their range front in Finland. The population of Finland is 100 years old and the population density is still decreased in compared to Central Europe. My hypothesis was that reed warblers would sing less complex songs at lower densities as competition for mates is low. I used song recordings of 51 reed warbler males, which I recorded during the breeding season 2019 in song complexity analyses. Recordings were from South Finland from 14 locations in 6 municipalities. Most of the recording places were coastal reedbeds. A couple of recording places were reedbeds in inland ponds and reedbeds in a flooded forest. I counted the male densities within 50 m and 200 m radius around the recorded males using Google Maps. I took account the males which were at least 20 m away from each other and were there in the same week than the recorded male. I extracted four within-song complexity variables from the recordings: Total number of syllables, number of syllable types, number of syllable transitions and mean frequency bandwidth. I measured song complexity with spectral analyses using Koe, a bioacoustic software. I did next analyses with R to see if there is association between local density and complexity. I found that one of the song complexity variables, total number of syllables, was significantly positively associated with the density of singing males within 200 m radius. The other variables were not significantly associated with male density, but number of syllable types and number of syllable transitions showed a similar relationship with singing male density within 200 m. Previous studies support the hypothesis that higher local density can increase some aspects of song complexity, for example number of syllables or length of phrases. Some males did not have neighbours within 50 m. Density within 50 m had no significant relationship with song complexity variables. However, mean frequency bandwidth, total number of syllables and number of syllable types showed more variation when neighbours were present, which suggest that males increase their effort when they have competition. Finally, I found that the number of syllable transitions decreased as the breeding season progressed. This could be explained with younger birds coming to breed later or with paired males singing less.