Pollutants increase song complexity and the volume of the brain area HVC in a songbird

Environmental pollutants which alter endocrine function are now known to decrease vertebrate reproductive success. There is considerable evidence for endocrine disruption from aquatic ecosystems, but knowledge is lacking with regard to the interface between terrestrial and aquatic ecosystems. Here, we show for the first time that birds foraging on invertebrates contaminated with environmental pollutants, show marked changes in both brain and behaviour. We found that male European starlings (Sturnus vulgaris) exposed to environmentally relevant levels of synthetic and natural estrogen mimics developed longer and more complex songs compared to control males, a sexually selected trait important in attracting females for reproduction. Moreover, females preferred the song of males which had higher pollutant exposure, despite the fact that experimentally dosed males showed reduced immune function. We also show that the key brain area controlling male song complexity (HVC) is significantly enlarged in the contaminated birds. This is the first evidence that environmental pollutants not only affect, but paradoxically enhance a signal of male quality such as song. Our data suggest that female starlings would bias their choice towards exposed males, with possible consequences at the population level. As the starling is a migratory species, our results suggest that transglobal effects of pollutants on terrestrial vertebrate physiology and reproduction could occur in birds.

Song as an honest signal of developmental stress in the zebra finch (Taeniopygia guttata)

In a wide range of bird species, females have been shown to express active preferences for males that sing more complex songs. Current sexual selection theory predicts that for this signal to remain an honest indicator of male quality, it must be associated with an underlying cost of development or maintenance. There has been considerable debate questioning the costs associated with song production and learning. Recently, the nutritional stress hypothesis proposed that song complexity could act as an indicator of early developmental history, since the song control nuclei in the brain are laid down early in life. Here we test the nutritional stress hypothesis, by investigating the effects of dietary stress on the quality of adult song produced. In addition, we tested the effects of elevated corticosterone during development on song production to test its possible involvement in mediating the effects of developmental stress. The results demonstrate that both dietary restriction and elevated corticosterone levels significantly reduced nestling growth rates. In addition, we found that experimentally stressed birds developed songs with significantly shorter song motif duration and reduced complexity. These results provide novel experimental evidence that complex song repertoires may have evolved as honest signals of male quality, by indicating early developmental rearing conditions.

Song and female choice for extrapair copulations in the sedge warbler, Acrocephalus schoenobaenus

Although 90% of passerine birds live in socially monogamous pair bonds, molecular studies have revealed that genetic polygamy occurs in 86% of surveyed passerines, because individuals engage in copulations outside the pair bond (extrapair copulations; EPCs). Most explanations for the occurrence of EPCs involve female gaining indirect benefits from the extrapair male. The sedge warbler is a socially monogamous species in which some offspring result from EPCs (8% in this study). Complex song is a sexually selected male trait used by females which select mates based on a variety of male qualities. We used microsatellite DNA profiling to detect extrapair young and assign paternity. ‘Good genes’ theory predicts that females should engage in EPCs with males of higher quality than their social mate, with resulting fitness benefits. Extrapair males had smaller song repertoires and smaller territories than the social mate. This apparent preference for small-repertoire males as extrapair mates conflicts with the predictions from previous studies of this species. Sudden cessation of song after pairing may mean that song cues are unavailable for later extrapair matings and females may switch to other cues. Such behaviour may lead to different patterns of female choice during social and extrapair mating in the sedge warbler. We conclude that multiple reasons underlie patterns of female choice in this species.

Song sharing and repertoire size in the sedge warbler, Acrocephalus schoenobaenus: changes within and between years

The complex song of the male sedge warbler functions mainly in sexual attraction and the evolution of repertoire size is driven primarily by female choice. As male song ceases upon pairing, male–male singing interactions are relatively brief and have not been studied to our knowledge. This study shows that young males in their first breeding season shared significantly more syllables with their nearest neighbour than with their fathers or more distant males. Moreover, daily recordings revealed that rapid learning and modification of syllable repertoires occurred, resulting in a progressive increase in sharing within just a few days. This does not lead to a gradual increase in repertoire size as some syllables are dropped and new ones are acquired. This turnover process allows males to share syllables with their neighbours, whilst repertoire size, known to be important in female choice, remains relatively constant in any one year. Individual males were followed for several years and also showed considerable syllable turnover between years. However, in this case, repertoire size was found to increase between years, the largest increase occurring between the first and second years. We obtained a significant positive correlation between repertoire size and age, suggesting that females choosing males with larger repertoires may gain indirect (genetic) benefits for their offspring, such as good genes for viability. Whilst these results reveal a more flexible picture of repertoire turnover than previously suspected, the relative stability of repertoire size within a season and the increase with age suggests that repertoire size remains a likely target for sexual selection by female choice.

The application of a fuzzy rule based system in an exposition of the antecedents of sedge warbler song flight

The utilization of a fuzzy aspect within data analysis attempts to move from a quantitative to a more qualitative investigative environment. As such, this may allow the more non-quantitative researchers results they can use, based on sets of linguistic terms. In this paper an inductive fuzzy decision tree approach is utilized to construct a fuzzy-rule-based system for the first time in a biological setting. The specific biological problem considered attempts to identify the antecedents (conditions in the fuzzy decision rules) which characterize the length of song flight of the male sedge warbler when attempting to attract a mate. Hence, for a non-quantitative investigator the resultant set of fuzzy rules allows an insight into the linguistic interpretation on the relationship between associated characteristics and the respective song flight duration.

Developmental stress selectively affects the song control nucleus HVC in the zebra finch brain

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.

Song as an honest signal of past developmental stress in the European starling (Sturnus vulgaris)

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'.

Parasites affect song complexity and neural development in a songbird

There is now considerable evidence that female choice drives the evolution of song complexity in many songbird species. However, the underlying basis for such choice remains controversial. The developmental stress hypothesis suggests that early developmental conditions can mediate adult song complexity by perturbing investment in the underlying brain nuclei during their initial growth. Here, we show that adult male canaries (Serinus canaria), infected with malaria (Plasmodium relictum) as juveniles, develop simpler songs as adults compared to uninfected individuals, and exhibit reduced development of the high vocal centre (HVC) song nucleus in the brain. Our results show how developmental stress not only affects the expression of a sexually selected male trait, but also the structure of the underlying song control pathway in the brain, providing a direct link between brain and behaviour. This novel experimental evidence tests both proximate and ultimate reasons for the evolution of complex songs and supports the Hamilton–Zuk hypothesis of parasite-mediated sexual selection. Together, these results propose how developmental costs may help to explain the evolution of honest advertising in the complex songs of birds.

Developmental stress, social rank and song complexity in the European starling (Sturnus vulgaris)

Bird song is a sexually selected trait and females have been shown to prefer males that sing more complex songs. However, for repertoire size to be an honest signal of male quality it must be associated with some form of cost. This experiment investigates the effects of food restriction and social status during development on song complexity in the European starling (Sturnus vulgaris). Birds that experienced an unpredictable food supply early in life produced a significantly smaller repertoire of song phrases than those with a constant food supply. Social status during development was also significantly correlated with repertoire size, with dominant birds producing more phrase types. This study therefore provides novel evidence that social as well as nutritional history may be important in shaping the song signal in this species.

Developmental stress affects the attractiveness of male song and female choice in the zebra finch (Taeniopygia guttata)

Developmental stress has recently been shown to have adverse effects upon adult male song structure in birds, which may well act as an honest signal of male quality to discriminating females. However, it still remains to be shown if females can discriminate between the songs of stressed and non-stressed males. Here we use a novel experimental design using an active choice paradigm to investigate preferences in captive female zebra finches (Taeniopygia guttata). Nine females were exposed to ten pairs of songs by previously stressed and non-stressed birds that had learned their song from the same tutor. Song pairs differed significantly in terms of song complexity, with songs of stressed males exhibiting lower numbers of syllables and fewer different syllables in a phrase. Song rate and peak frequency did not differ between stressed and non-stressed males. Females showed a significant preference for non-stressed songs in terms of directed perching activity and time spent on perches. Our results therefore indicate that developmental stress affects not only the structure of male song, but that such structural differences are biologically relevant to female mate choice decisions.

Singing as a handicap : the effects of food availability and weather on song output in the Australian reed warbler Acrocephalus australis

Bird song is generally regarded as a sexually selected trait, and may represent a reliable handicap signal under at least certain conditions. Females may use the degree of male song production as a reliable cue to male condition or territory quality. We investigated the effect of supplementary feeding on song output in the migratory Australian reed warbler Acrocephalus australis. We experimentally increased the food availability on alternate days, and recorded several weather variables. We measured song rate and song length independently. Supplementary fed birds sang more on feeding days than on non-feeding days, while control birds did not show this effect. Song output was not significantly associated with any of the weather variables examined. Our results indicate that singing has the potential to serve as a reliable handicap signal to territorial food availability irrespective of the prevailing weather conditions. We discuss the role of energetic constraints and behavioural flexibility on the signaling function of song.

Owl song : a study in tragedy

The thesis is a creative work with exegesis. The creative component is a playscript exploring European tragic form within a mythologised Australian context. The exegesis examines the research material that informed the playwriting process; including tragic theory, dramatic character, place and landscape, and the use of landscape in theatre.