Marta Borowiec

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.

Short records of marsh warbler (Acrocephalus palustris) song provide indices that correlate with nesting success

The migrant Marsh Warbler (Acrocephalus palustris) has a complex song repertoire, but such complexity makes quantitative comparison of songs between individual males both time consuming and challenging. We investigate a streamlined method of song analysis that uses 2-min records of song to provide simpler relative indices of repertoire size, including the use of capture-recapture and species-richness models. For each male, three attributes of song were determined: the song complexity, the total repertoire elements and the estimated repertoire size based on the Burnham and Overton jackknife method. Males with higher song indices tend to have greater nesting success, suggesting that even short records of song can indicate male quality to prospective female mates. Why should male Marsh Warblers have long and sustained songs when only 2-min records correlate with nesting success? Assuming that song advertises the quality of the male, and that the quality of such advertisement is sustained throughout the male’s song period, we argue that the female may need to have only brief exposure initially to that song to assess its quality and hence the quality of the male. More continuous periods of song may reinforce that initial choice by the female, and allow males to remain conspicuous to transient females moving around the territories of potential male mates.

Male whitethroats, Sylvia communis, advertise their future contribution to parental care

Summary In altricial species participation of males in parental care enhances reproductive success of females. How does a female select a mate who will allocate time and energy to parental effort? In the whitethroat Sylvia communis, a socially monogamous bird, parental performance of males might be predicted on the basis of elaborated song flights displayed in courtship. The correlation analysis revealed that males which advertised intensively needed less time to attract a female and also their parental performance was better compared to males that produced cheaper signals. In the subsequent experiment, we handicapped males by increasing their body mass by 5% with a weight attached to tail feathers. Males in the treatment group reduced the proportion of showy song flights significantly more than controls and their mating success was significantly lower. We conclude that song flights in whitethroats honestly signal male quality and that the signalling system depends on a dynamic handicap trait which responds to a relatively small change in the male.

Factors Driving Variation in Biparental Incubation Behaviour in the Reed Warbler Acrocephalus scirpaceus

Biparental incubation is a common pattern of parental care in birds. Within species with biparental incubation, the contribution of each sex can vary widely. Many studies have addressed the factors that influence variation in female incubation behaviour, but the underlying causes of within-species variation in male incubation behaviour remain poorly understood. In this paper we analyse incubation behaviour in the Reed Warbler, a small, predominantly socially monogamous passerine nesting in reed beds. We examined the impact of time of day, weather conditions (ambient temperature, wind speed, rainfall) and progress of the breeding season on male and female incubation behaviour in 81 pairs of Reed Warbler breeding in the Barycz Valley (SW Poland). We found that females had on average higher nest attentiveness (total time spent incubating per hour) than their partners (47% vs. 29%) but mean incubation bout length (a single, uninterrupted stay at the nest) did not differ significantly between the sexes (9 min vs. 7 min). The two parents responded differently to changing environmental parameters. Female nest attentiveness was unaffected by date, time of the day, advancement of incubation and weather conditions, while males spent more time on the nest at higher wind speeds and lower temperatures. In contrast, male incubation bout length was not affected by these factors, whereas female bout length increased throughout the breeding season and was longer at lower temperatures. Incubation recesses (periods where both parents were off the nest) were longer during favourable weather conditions (at high temperatures and low wind), probably because cooling of the eggs takes much longer under such conditions and parents can spend more time foraging. A comparison of our results with those from other populations revealed important betweenpopulation and sex-specific differences in nest attentiveness expressed as the percentage of male/female time spent on the nest. We suggest that betweenpopulation variation may result from differences in habitat quality and/or food resources affecting the necessity of male contribution to parental care, variation in breeding synchrony and densities influencing male engagement in extra-pair copulations, differences in predation levels, or methodological differences.

Song repertoire is not affected by stress in an adult male songbird, the Whitethroat Sylvia communis

Summary It is believed that bird song has evolved as a reliable signal of quality of displaying individuals. Recent research has focused on costs of development of complex song. In the present paper we test if the acquired repertoire size is costly to maintain. We compared changes in song structure in male Whitethroats (Sylvia communis) after 48 h exposure to a stressor (5% body mass weight attached to the tail feathers) vs. changes observed within the same time interval in the control group. The strophe length was marginally significantly shorter in the handicapped males comparing to controls. However, the repertoire size (i.e., a measure of diversity of different song elements) remained intact in both groups. We concluded that the song repertoire in Whitethroats is a static secondary sexual trait. A review of literature has revealed no convincing examples of decreasing repertoire size in adult male songbirds. Further research is needed to improve our understanding of evolutionary and proximate mechanisms maintaining the stability of song repertoires.

Does heavy metal exposure affect the condition of Whitethroat ( Sylvia communis ) nestlings

Anthropogenic pollution results in high concentrations of heavy metals in the environment. Due to their persistence and a high potential for bioaccumulation, metals are a real threat for birds breeding in industrial areas. The aim of the present study has been to explore the contents of heavy metals (arsenic As, cadmium Cd, chromium Cr, copper Cu, iron Fe, nickel Ni, lead Pb and zinc Zn) in the excreta of Whitethroat (Sylvia communis) nestlings living in polluted environment and to investigate the relationship between these contents and the nestlings’ condition. Excrement samples contained all the studied elements. The contents of arsenic, cadmium, copper and zinc in the excreta of nestlings from nests located close to a slag dump were several times higher than in the soil near the dump, which suggested accumulation in food consumed by the birds. Condition parameters (body mass and haemoglobin concentration) were not related to heavy metal concentrations in the nestlings’ excreta, except of Zn. It is possible that Whitethroats are able to detoxicate heavy metals to a certain extent. Detailed, multi-element analysis of the environment, food and bird tissues or excreta should be performed to explore relations between different chemicals and bird condition.