Tudor I. Draganoiu

In a songbird, the black redstart, parents use acoustic cues to discriminate between their different fledglings

Several studies on parental investment in territorial songbirds have reported the existence of brood division. This is a type of postfledging care in which each parent has long-term feeding preferences for different young within a brood, creating two family units. Recent theoretical work indicates that conflicts between individuals should select for brood division. However, little is known about the mechanisms involved in the onset and maintenance of this behavioural strategy. Given the high rate of fledglings’ begging calls, we hypothesized that acoustic discrimination could explain the stability of feeding preferences at a proximate level. In a 3-year field study, we recorded the responses of parent black redstarts, Phoenicurus ochruros, a territorial songbird, to playback of the begging calls of fledglings fed by the male and by the female. Parents responded more to the calls of the fledglings that they preferentially fed. A principal component analysis of the calls suggested that parents may recognize individual offspring. To our knowledge, this study provides the first evidence that a bird can acoustically discriminate between two categories of its own offspring: those that it preferentially feeds and those fed by the other parent.

Urban noise undermines female sexual preferences for low-frequency songs in domestic canaries

Increasing levels of anthropogenic noise represent a challenge for animals living in urban areas and birds, especially, may suffer from noisy conditions as they use singing to attract mates. Most anthropogenic noise is low in frequency and singing at high frequencies under noisy urban conditions may avoid masking and thus be a good strategy for breeding success in cities. Despite comparative, correlational and some experimental studies supporting this hypothesis, empirical studies on the impact of noise on sexual behaviour are largely lacking. Domestic canaries, Serinus canaria, provide an excellent model system to test unequivocal sexual responsiveness as receptive females perform copulation solicitation displays (CSD) to male song. We have previously shown that CSD rate was higher for low- than for high-frequency songs. In the current study, we tested whether a typical urban noise spectrum, with a bias towards low frequencies, could undermine sexual preferences. Using overlapping and alternating noise exposure regimes while broadcasting male songs we found that masking by urban noise reduced female responsiveness to low-frequency attractive songs. Under the same conditions the responsiveness to high-frequency songs remained unaffected and, consequently, the sexual preference for low- over high-frequency songs had faded because of the urban noise. We discuss to what extent our results can be extrapolated to other wild bird species and speculate about the adaptive value of the typical upward shift in frequency found in many city birds.

Estimating the complexity of bird song by using capture-recapture approaches from community ecology

Repertoire size, the number of unique song or syllable types in the repertoire, is a widely used measure of song complexity in birds, but it is difficult to calculate this exactly in species with large repertoires. A new method of repertoire size estimation applies species richness estimation procedures from community ecology, but such capture-recapture approaches have not been much tested. Here, we establish standardized sampling schemes and estimation procedures using capture-recapture models for syllable repertoires from 18 bird species, and suggest how these may be used to tackle problems of repertoire estimation. Different models, with different assumptions regarding the heterogeneity of the use of syllable types, performed best for different species with different song organizations. For most species, models assuming heterogeneous probability of occurrence of syllables (so-called detection probability) were selected due to the presence of both rare and frequent syllables. Capture-recapture estimates of syllable repertoire size from our small sample did not differ significantly from previous estimates using larger samples of count data. However, the enumeration of syllables in 15 songs yielded significantly lower estimates than previous reports. Hence, heterogeneity in detection probability of syllables should be addressed when estimating repertoire size. This is neglected using simple enumeration procedures, but is taken into account when repertoire size is estimated by appropriate capture-recapture models adjusted for species-specific song organization characteristics. We suggest that such approaches, in combination with standardized sampling, should be applied in species with potentially large repertoire size. On the other hand, in species with small repertoire size and homogenous syllable usage, enumerations may be satisfactory. Although researchers often use repertoire size as a measure of song complexity, listeners to songs are unlikely to count entire repertoires and they may rely on other cues, such as syllable detection probability.

Parental care and brood division in a songbird, the black redstart

Sexual conflict over parental care can be mediated through differences in male and female overall feeding rates, brood division or both. At present, it is not clear whether post-fledging brood division occurs due to sexual conflict over parental investment or is due to bi-parental cooperation, e.g. increase offspring fitness. We provide evidence suggesting that brood division in the black redstart, Phoenicurus ochruros is due to sexual conflict. Males and females had similar feeding contributions during the nestling stage, which is common for most passerine species. After fledging, each parent showed long-term feeding preferences for particular chicks within the brood. In most cases (74%; 17/23) both parents provided care but males tended to feed less fledglings than females did and in about a quarter of cases (26%; 6/23) females fed the whole brood by themselves. The relative amount of male to female post-fledging feedings showed a significant negative relationship with the proportion of fledglings cared for exclusively by the male. These results suggest (1) a close link between the amount of parental care and brood division; (2) sexual conflict can be mediated through brood division; (3) female redstarts appear to loose this conflict more often than male redstarts, with in the extreme cases males showing post-fledging brood desertion. A literature review shows brood division to occur in at least a dozen of songbird species but male black redstarts have the lowest relative post-fledging parental investment, expressed either as feeding rates or number of chicks in care.

Long-term effect of isolation rearing conditions on songs of an ‘open-ended’ song learner species, the canary

Among songbird species, the canary, Serinus canaria, is considered to be an ‘open-ended’ song learner because adults continue to change their syllable repertoire throughout their lives. However, it remains unclear whether the capacity to alter the syllable repertoire allows birds to acquire new song material to correct abnormal song characteristics that result from auditory isolation during development. To address this question, we reared males without any experience of typical adult conspecific song models from fledging to sexual maturity. They were then placed for at least 2 years in an aviary that included adult conspecific males. Discriminant function analyses on songs indicated that features describing the fine structure of syllables showed significant differences between isolation-reared and control birds. Analysis of syllable repertoires attributed differences to a higher proportion of multiple-element syllables in songs of isolation-reared males than in songs of control males. Isolation-reared birds were able to modify their syllable repertoire throughout their lives. However, the proportion of multiple-element syllables, instead of decreasing during the years spent in the aviary, continued to form a large part of the songs. The ability of isolation-reared canaries to change their repertoires failed to rectify, at least in large part, the syllable abnormalities caused by the lack of a song model during their first year of life. Therefore, new and previously acquired syllables might result from similar underlying processes that would set limits to the kinds of syllables produced in adulthood.

Studying Female Reproductive Activities in Relation to Male Song: The Domestic Canary as a Model

Abstract Birdsong in oscine birds serves both intrasexual and intersexual functions. The aim of this chapter is to contribute to a better understanding of how birdsong is involved in female reproductive activity using the domestic canary as a model. Some special song phrases containing bipartite syllables composed of abrupt frequency falls and short silences (sexy phrases) appear to be particularly efficient to elicit sexual responses. Females canaries seem predisposed to prefer sexy phrases even though early or adult acoustic experience can affect this phenomenon. Moreover, eavesdropping on singing interactions, previous reproductive experience, as well as physical condition may be involved in shaping females’ preferences. Repeated exposures to male sexy phrases during reproduction do not influence the development of nest building or egg laying but affect egg quality. From the sender perspective, the production of the complex sexy phrases needs particular skills from the peripheral motor level as well as the central nervous system level.

Song stability and neighbour recognition in a migratory songbird, the black redstart

Territorial male songbirds have the ability to discriminate between the songs of their neighbours and those of strangers and for a few species it has been shown that they maintain this ability from one breeding season to the next. To better understand the acoustic basis of this long-term discrimination ability we studied song stability across two breeding seasons in a migratory songbird with high inter-annual return rates and territory stability, the black redstart, Phoenicurus ochruros. Strophe repertoires of 14 males (≥2 years old) were stable from one breeding season to the next and high strophe sharing occurred for males within the same group of houses or hamlets (81%) in contrast to only limited sharing between different hamlets (15%). However, subtle differences exist between the renditions of the same strophe sung by neighbouring males and these differences equally show an inter-annual stability, providing an acoustic basis for long-term discrimination abilities. Playback tests showed the existence of a strong dear-enemy effect: males reacted less aggressively to the familiar, often shared song of a neighbour than to a stranger unshared song and this pattern was maintained when birds returned from migration one year later. We discuss on one side the possible mechanisms leading to the observed patterns of song sharing and on the other side the significance of stable vocal signatures for neighbour recognition.

Individual benefits of nestling begging: experimental evidence for an immediate effect, but no evidence for a delayed effect

The evolutionary stability of honest signalling by offspring is thought to require that begging displays be costly, so the costs and benefits of begging—and whether they are experienced individually or by the whole brood—are crucial to understanding the evolution of begging behaviour. Begging is known to have immediate individual benefits (parents distribute more food to intensely begging individuals) and delayed brood benefits (parents increase provisioning rate to the brood), but the possibility of delayed individual benefits (previous begging affects the current distribution of food) has rarely, if ever, been researched. We did this using playback of great tit Parus major chick begging and a control sound from either side of the nest. Male parents fed chicks close to the speaker more when great tit chick begging, but not other stimuli, was played back. In contrast, there was no effect of playback at the previous visit on the chicks that male parents fed. We have thus demonstrated an immediate individual benefit to begging, but found no evidence of a delayed individual benefit in this species.