| Summary: | 4 Abstract Bird song is usually considered to have two primary functions; attract females and repels males. Hence, song features can be an indicator of male quality or aggressive motivation. It has been suggested that one of those features could be in the form of a trill - physically demanding structures. Two types of trill has been found in tree pipit, Anthus trivialis, "I" and "M" trills. Trill rates significantly differ among males in this species therefore we assume that repetitive syllable production may indicate male quality. Firstly, we tested if both types of trills are used in aggressive context using playback experiments. Only "I" trills were sung by males in response to playback, regardless of the type of trill involved in playback. Moreover, males increased the frequency of the singing of that trill as an immediate response to playback, and also increased the trill rate in comparison with spontaneous songs. In addition males who sung faster "I" trills were able to maintain the territory. We conclude "I" trills may carry information about the signaller's aggressiveness. Surprisingly, production of "M" trills decreased after the simulated territorial intrusion. Secondly, we tested hypotheses that males modify their response according to the intruders' quality therefore we used playbacks containing...
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