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An attempt has been made to explain why the songs of Acrocephalus warblers have become so complex during their evolution. A considerable body of evidence has implicated sexual selection, and two predictions from sexual selection theory have been tested. First, males with more complex songs should attract females for pairing before their rivals with simpler songs. This prediction has been confirmed, when a strong inverse correlation between syllable repertoire size (song complexity) and pairing date (female choice) was obtained in a population of sedge warblers. No significant correlation was obtained between male arrival date and pairing date. Assuming that the first (usually older) males back should select the best territories, then territory quality appeared to have little or no significant effect upon female choice. The second prediction tested was that due to increased sexual selection pressure, males of polygynous species should have evolved more comples songs than males of monogamous species. Of the six common European Acrocephalus species studied, four are monogamous and two polygynous. The prediction was not upheld, as the four monogamous species had long, complex and variable songs, and the two polygynous species had shorter, simpler and less variable songs. The explanation for this apparent paradox can be found in the different relationships between female choice, feeding ecology and parental investment in monogamous or polygynous Acrocephalus species. In polygynous species, a female must ensure the territory contains enough resources to enable her to feed the young and should base her choice primarily upon territory quality. In monogamous species, a female must select a male who will help her to feed the young, and should base her choice more directly upon male quality. The songs of polygynous species have therefore evolved primarily through indirect (intrasexual) sexual selection pressure, which has produced shorter, simpler and more stereotyped songs more suited to male-male interactions and a territorial function. The songs of monogamous species have evolved primarily through direct (intersexual) selection pressure, which has produced the long, complex and variable songs which females find more attractive.

Bird song is a sexually selected male trait where females select males on the basis of song quality. It has recently been suggested that the quality of the adult male song may be determined by nutritional stress during early development. Here, we test the 'nutritional-stress hypothesis' using the complex song of the European starling. Fledgling starlings were kept under experimental treatment (unpredictable short-term food deprivations) or control conditions (ad libitum food supply), for three months immediately after independence. We measured their physiological and immune responses during the treatment and recorded song production during the following spring. Birds in the experimental group showed increased mass during the treatment and also a significantly suppressed humoral response compared with birds in the control group. There was no difference between the groups in the cell-mediated response. Next spring, males in the experimental group spent less time singing, sang fewer song bouts, took longer to start singing and also sang significantly shorter song bouts. These data support the hypothesis that both the quality and quantity of song produced by individual birds reflect past developmental stress. The results also suggest the 'nutritional-stress hypothesis' is best considered as a more general 'developmental-stress hypothesis'.

Bird song is one of the most remarkable and impressive sounds in the natural world, and has inspired not only students of natural history, but also great writers, poets and composers. Extensively updated from the first edition, the main thrust of this book is to suggest that the two main functions of song are attracting a mate and defending territory. It shows how this evolutionary pressure has led to the amazing variety and complexity we see in the songs of different species throughout the world. Writing primarily for students and researchers in animal behavior, the authors review over 1000 scientific papers and reveal how scientists are beginning to unravel and understand how and why birds communicate with the elaborate vocalizations we call song. Highly illustrated throughout and written in straightforward language, Bird Song also holds appeal for amateur ornithologists with some knowledge of biology.

Songbirds sing complex songs as a result of evolution through sexual selection. The evolution of such sexually selected traits requires genetic control, as well as selection on their expression. Song is controlled by a discrete neural pathway in the brain, and song complexity has been shown to correlate with the volume of specific song control nuclei. As such, the development of these nuclei, in particular the high vocal centre (HVC), is thought to be the mechanism controlling signal expression indicating male quality. We tested the hypothesis that early developmental stress selectively affects adult HVC size, compared with other brain nuclei. We did this by raising cross-fostered zebra finches (Taeniopygia guttata) under stressed and controlled conditions and determining the effect on adult HVC size. Our results confirm the strong influence of environmental conditions, particularly on HVC development, and therefore on the expression of complex songs. The results also show that both environmental and genetic factors affect the development of several brain nuclei, highlighting the developmental plasticity of the songbird brain. In all, these results explain how the complex song repertoires of songbirds can evolve as honest indicators of male quality.

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SUMMARYRecent models of animal signalling emphasize the evolution of complex displays containing ' multiple messages '. A variety of potential cues used in female choice were investigated during a three-year field study of the sedge warbler, Acrocephalus schoenobaenus. Twelve possible cues were investigated, and three were found to have a significant influence upon pairing date. Two were different measures of song (repertoire size and song flighting) and one a measure of territory (territory size). Repertoire and territory size had a significant influence on pairing date in all three years, and song-flighting in two. The three cues were not intercorrelated and so had independent effects upon pairing date. We suggest that females select males upon multiple cues as these reflect different aspects of male and territory quality.

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