Katharina Mahr

Female attractiveness affects paternal investment: experimental evidence for male differential allocation in blue tits

IntroductionThe differential allocation hypothesis (DAH) predicts that individuals should adjust their parental investment to their current mate’s quality. Although in principle the DAH holds for both sexes, male adjustment of parental investment has only been tested in a few experimental studies, revealing contradictory results. We conducted a field experiment to test whether male blue tits (Cyanistes caeruleus) allocate their parental effort in relation to female ornamentation (ultraviolet colouration of the crown), as predicted by the DAH.ResultsWe reduced the UV reflectance in a sample of females and compared parental care by their mates with that of males paired to sham-manipulated control females. As predicted by the DAH our results demonstrate that males paired with UV-reduced females invested less in feeding effort but did not defend the chicks less than males paired with control females.ConclusionsTo our knowledge, this is one of the first studies providing support for male differential allocation in response to female ornamentation.

When subspecies matter: resident Superb Fairy-wrens (Malurus cyaneus) distinguish the sex and subspecies of intruding birds

Abstract The widely accepted functions of complex bird song—to defend a territory or attract a mate, or both—have generally been tested in northern hemisphere species in which males produce the song and females choose the singer. In our study species, the Superb Fairy-wren (Malurus cyaneus), both males and females sing a solo song throughout the year. We compare the chatter song in males and females of two genetically distinct subspecies, and test if resident birds respond to the sex and subspecies of the intruder song. Compared with island birds (M. c. ashbyi), mainland Superb Fairy-wrens (M. c. leggei) produced songs with lower frequency and fewer elements. Compared with females, males produced longer songs with more elements. Resident birds showed acoustical discrimination for the sex and subspecies of the intruder bird. The response of resident pairs was positively correlated, but each sex showed a solo response. Resident males were the first to respond to male intruders, and resident females were the first to respond to female intruders. Fairy-wrens had the strongest response towards (1) intruders of the same subspecies and (2) male intruders. The finding of signal divergence and acoustical discrimination in males and females makes this a model system to test the mechanism of reproductive isolation when both sexes sing.

Female in-nest chatter song increases predation

Female song is an ancestral trait in songbirds, yet extant females generally sing less than males. Here, we examine sex differences in the predation cost of singing behaviour. The superb fairy-wren (Malurus cyaneus) is a Southern Hemisphere songbird; males and females provision the brood and produce solo song year-round. Both sexes had higher song rate during the fertile period and lower song rate during incubation and chick feeding. Females were more likely than males to sing close to or inside the nest. For this reason, female but not male song rate predicted egg and nestling predation. This study identifies a high fitness cost of song when a parent bird attends offspring inside a nest and explains gender differences in singing when there are gender differences in parental care.

Parental risk management in relation to offspring defence: bad news for kids

Do parents defend their offspring whenever necessary, and do self-sacrificing parents really exist? Studies recognized that parent defence is dynamic, mainly depending on the threat predators pose. In this context, parental risk management should consider the threat to themselves and to their offspring. Consequently, the observed defence should be a composite of both risk components. Surprisingly, no study so far has determined the influence of these two threat components on parental decision rules. In a field experiment, we investigated parental risk taking in relation to the threat posed to themselves and their offspring. To disentangle the two threat components, we examined defence behaviours of parent blue tits Cyanistes caeruleus towards three different predators and during different nestling developmental stages. Nest defence strategies in terms of alarm call intensity and nearest predator approach differed between the three predators. Defence intensity was only partly explained by threat level. Most importantly, parental risk management varied in relation to their own, but not offspring risk. Parent defence investment was independent of nestling risk when parents followed a high-risk strategy. However, parents considered nestling as well as parental risk when following a low-risk strategy. Our findings could have general implications for the economy of risk management and decision-making strategies in living beings, including humans.

Olfactory Assessment of Competitors to the Nest Site: An Experiment on a Passerine Species

Since most avian species have been considered anosmic or microsmatic, olfaction and associated behavioural patterns have hardly been investigated. Most importantly, empirical data on avian olfaction is not equally distributed among species. Initial investigations focused on species with relatively big olfactory bulbs because they were thought to have better olfactory capabilities. Hence, in this study we tested the ability of house sparrows (Passer domesticus) to use chemical cues as parameters to estimate nest features. House sparrows are a commonly used model species, but their olfactory capabilities have not been studied so far. We offered two different odours to males and females, namely the scent of mouse urine (Mus musculus domesticus), representing a possible competitor and a threat to eggs and hatchlings, and the odour of hay, representing a familiar and innocuous odour. The experiment was performed at the sunset to simulate a first inspection to new possible roosting or nesting sites. Interestingly, males but not females preferred to spend significantly more time in front of the hay odour, than in front of the scent of mouse urine. Our results strengthen the hypothesis that oscines can not only perceive odours but also use olfaction to assess the environment and estimate nest site quality.

Distinct colour morphs in nestling European Bee-eaters Merops apiaster: is there an adaptive value?

In a few bird species, dimorphism already exists in nestling and juvenile plumage coloration and these colour morphs are often attributable to different sexes. In this study we detected variation in nestling coloration among European Bee-eaters Merops apiaster. We identified two distinct colour morphs, namely nestlings with yellowish-brown and nestlings with green back feathers. By means of genetic methods, we determined nestling sex. It turned out that the back colour is a significant indicator for sex. Male nestlings have yellowish-brown and females green back feathers, but there are exceptions. Population sex ratio was about equal but we found sex-biased variation in several nests. Furthermore, we found evidence that colour is an indicator for condition especially in those individuals where sex and coloration do not match.ZusammenfassungFarbddimorphismus im Gefieder von Nestlingen kennt man von einigen Vogelarten. Häufig ist dieser Farbdimorphismus geschlechtsspezifisch. Wir fanden einen solchen Farbdimorphismus bei Europäischen Bienenfressern Merops apiaster. Nestlinge haben entweder braune oder grüne Rückenfedern. Mit Hilfe genetischer Methoden haben wir das Geschlecht bestimmt. Die Ergebnisse zeigen eine hohe aber nicht vollständige Übereinstimmung zwischen Geschlecht und Gefiederfärbung. Das Geschlechterverhältnis in unserer Population scheint ausgewogen obwohl einzelne Nester in beide Richtungen geschlechtsverschoben sein können. Unsere Ergebnisse deuten weiters darauf hin, dass Gefiederfärbung auch mit der Kondition zusammenhängt, speziell bei den Individuen bei denen Geschlecht und Färbung nicht übereinstimmen.

Multiple Ornaments—Multiple Signaling Functions? The Importance of Song and UV Plumage Coloration in Female Superb Fairy-wrens (Malurus cyaneus)

Showy ornaments are considered as outcomes of sexual selection processes. They provide a “badge of status” to impress conspecific rivals or potential mating partners. Single ornaments may signal attractiveness or individual quality, yet many species display multiple ornaments. There are several hypotheses that explain the existence of multiple ornaments, suggesting that different ornaments serve as different information sources. They may provide either additive or redundant information on the same quality traits, or are simply evolutionary leftovers with no further relevant information. Although females of many species display elaborated traits, most studies regarding multiple ornaments focus on males. However, given that in many species females also display multiple ornaments, the question about their functional significance arises. To understand the existence of female multiple ornaments we investigated ornamental features of female Superb Fairy-wrens (Malurus cyaneus), focusing on song and variation in plumage characteristics. Female Superb Fairy-wrens produce complex solo songs, for territorial defense, and have bright blue tail feathers. We examined the relationships between song and plumage coloration characteristics in relation to female quality parameters to investigate whether, and to what extent existing hypotheses on multiple ornaments in males may also apply to females. Based on song recordings and spectrometric measurements of UV-coloration of tail feathers, we derived a series of different song and plumage parameters. Our results indicate interrelationships between the song length (total number of elements in female song) and female body size, but not UV-coloration. Interestingly, song complexity (number of different elements in female song) did not correlate with morphological parameters, UV-Chroma and song length respectively. This suggests that i) song and plumage characteristics evolved independently and ii) even within one trait, namely song, multiple signalling should be considered. To our knowledge, this is the first study investigating multiple traits in female songbirds, raising the idea that multiple signalling of sexually selected traits is not restricted to males only.

Measuring short-term stress in birds: Comparing different endpoints of the endocrine-immune interface

Stress is a collective term for certain conditions and sequences of physiological events enabling living organisms to overcome unpredictable and uncontrollable situations. The context-dependent nature, multidimensional course and large individual variability make stress responses difficult to measure. In avian species, a plethora of studies on short-term stress responses have been conducted by measuring the corticosteroid response to a standardized stress protocol. Here we aimed to test the viability of the leukocyte coping capacity (LCC), measuring oxygen radical production by leukocytes, to assess short-term stress in birds. We collected blood samples from captive house sparrows (Passer domesticus) in the two seasons of winter and spring, right after capture and 30min thereafter. In order to assess the overall physiological stress response to a standardized stressor, i.e. handling and temporary constraint, we measured LCC and additionally combined it with measures of total circulating glucocorticoids (GCs) and oxidative stress. All three methodologies detected significant changes due to the stressor albeit they were not correlated with each other and revealed different information. There was no significant change in oxidative stress levels between the two time points although the amount of relative oxidative damage as well as the anti-oxidative capacity changed significantly. We observed a significant seasonal difference in GC stress response with no difference between sexes. On the contrary, LCC measures revealed with a high individual consistency, that individuals experienced a similar magnitude of stress in both seasons with a significant difference between sexes. Total GC-levels have to be interpreted with caution regarding the assessment of short-term stress reactions. We therefore suggest to supplementary combine classical approaches for measuring stress with the immunological tool of LCC. Our results reveal LCC as a strong and reliable tool to assess short-term stress in captive house sparrows and as promising for other bird species. Collectively the study highlights the necessity to incorporate a range of physiological systems and their endpoints to measure and to assess stress reactions effectively.

Experimental evidence for beneficial effects of projected climate change on hibernating amphibians.

Amphibians are the most threatened vertebrates today, experiencing worldwide declines. In recent years considerable effort was invested in exposing the causes of these declines. Climate change has been identified as such a cause; however, the expectable effects of predicted milder, shorter winters on hibernation success of temperate-zone Amphibians have remained controversial, mainly due to a lack of controlled experimental studies. Here we present a laboratory experiment, testing the effects of simulated climate change on hibernating juvenile common toads (Bufo bufo). We simulated hibernation conditions by exposing toadlets to current (1.5 °C) or elevated (4.5 °C) hibernation temperatures in combination with current (91 days) or shortened (61 days) hibernation length. We found that a shorter winter and milder hibernation temperature increased survival of toads during hibernation. Furthermore, the increase in temperature and shortening of the cold period had a synergistic positive effect on body mass change during hibernation. Consequently, while climate change may pose severe challenges for amphibians of the temperate zone during their activity period, the negative effects may be dampened by shorter and milder winters experienced during hibernation.

Female and male Blue Tits (Cyanistes caeruleus) sing in response to experimental predator exposition

Female song is recognized to serve a similar function as male song and underlies sexual selection processes; yet certain patterns of the expression of female singing behaviour are not in line with traditional explanations known from male songbirds. In particular, in northern hemisphere songbirds, female singing behaviour is regarded to occur only rarely, and; therefore, studies investigating it are sparse. Within the framework of an experimental study on nest defence behaviour, we observed female singing in a common European passerine, namely, the Blue Tit (Cyanistes caeruleus). Females expressed singing behaviour when a dummy of a Sparrow Hawk was exposed to the nest, raising the idea that song might be multifunctional in this species.ZusammenfassungGesangsverhalten von weiblichen und männlichen Blaumeisen (Cyanistes caeruleus) bei Prädationsexperimenten Es wird allgemein angenommen, dass die Funktionen von Weibchengesängen denen der Männchen sehr ähnlich sind und sie primär dem Territorial- und Partnerwahlverhalten (intra- und intersexuelle Selektion) dienen. Männchen und Weibchen unterscheiden sich aber aufgrund ihrer Physiologie und auch ihrer Reproduktionskosten. Man würde daher erwarten, dass vor allem Weibchen, ihre Gesänge vielseitiger einsetzen. Besonders bei Singvögeln der nördlichen Hemisphäre wurde weiblicher Gesang bis heute nur selten dokumentiert. Im Rahmen einer experimentellen Studie beschreibt die vorliegende Arbeit erstmals das Auftreten weiblichen Gesanges bei Blaumeisen (Cyanistes caeruleus) in der Gegenwart einer Sperberattrappe. Diese Beobachtung unterstützt die Hypothese, dass Weibchengesänge in mehreren und auch anderen als den hauptsächlich erwähnten und von Männchen bekannten Funktionen eingesetzt werden können.