Staffan Andersson

Ultraviolet colour variation influences blue tit sex ratios

Brilliant blue and violet structural colours are common plumage ornaments in birds, but their signalling functions are poorly understood. This may be because birds also communicate in ultraviolet (UV-A) wavelengths (320–400u2009nm), invisible to humans, but a strong spectral component of many structural colours. From a wild population of blue tits—Parus caeruleus, sexually dimorphic primarily in the ultraviolet—we report experimental evidence that females skew the sex ratio of their offspring in response to the ultraviolet plumage ornamentation of their mates. Masking male ultraviolet reflectance reversed a positive correlation between reflectance and brood sex ratio observed in control pairs, demonstrating a causal effect of male ultraviolet ornamentation on offspring sex ratio. Ultraviolet reflectance also predicted male survival to the following breeding season, suggesting that it serves as a viability indicator. When taken together with ecological effects (laying date, nesting area), our experiments reveal that an unexpected amount of control exists over the primary sex ratio in birds, suggesting that chromosomal sex determination may not constrain the sex ratios of multiparous vertebrates.

Ultraviolet colour vision and ornamentation in bluethroats

Many birds see in the ultraviolet (300–400 nm), but there is limited evidence for colour communication (signalling by spectral shape independently of brightness) in this ‘hidden’ waveband. Such data are critical for the understanding of extravagant plumage colours, some of which show considerable UV reflectance. We investigated UV colour vision in female social responses to the male UV/violet ornament in bluethroats, Luscinia s. svecica. In an outdoor aviary at the breeding grounds, 16 females were each presented with a unique pair of males of equal age. In UVR (UV reduction) males, sunblock chemicals reduced only the UV reflectance and thereby the spectral shape (colour) of the throat ornament. In NR (neutral reduction) males, an achromatic pigment in the same base solvent (preen gland fat) was used for a corresponding but uniform brightness reduction. Both colour and brightness effects were invisible to human eyes, and were monitored by spectrometry. In 13 of the 16 trials, the female associated most with the NR male, a preference that implies that UV colour vision is used in mate choice by female bluethroats. Reflectance differences between one–year–old and older males were significant only in UV, suggestive of a UV colour cue in age–related mate preferences.

Multiple receivers, multiple ornaments, and a trade-off between agonistic and epigamic signaling in a widowbird

Sexual displays often involve several different ornamental traits. Yet most indicator models of sexual selection based on a single receiver (usually a choosy female) find that multiple handicap signals should be unstable. Here we study reasons for this contradiction, analyzing signal function, signal content, and trade‐offs between signals in the polygynous red‐collared widowbird Euplectes ardens. Males have both a long, graduated tail and a red carotenoid collar badge. Territory‐holding “residents” have slightly shorter tails than the nonbreeding “floaters,” but their carotenoid collars are 40% larger, and they have (on the basis of reflectance spectrometry and objective colorimetry) a 23‐nm more long‐wave (“redder”) hue than floaters. This corroborates experimental evidence that the red collar is selected by male contest competition, whereas female choice is based almost exclusively on male tail length. Tail length is negatively correlated with the carotenoid signal, which together with body size and condition explains 55% of the variation in tail length. The trade‐off in tail length and carotenoid investment is steeper among residents, suggesting an interaction with costs of territory defense. We propose that the “multiple receiver hypothesis” can explain the coexistence of multiple handicap signals. Furthermore, the trade‐off between signal expressions might contribute to the inverse relation between nuptial tail elongation and coloration in the genus Euplectes (bishops and widowbirds).

Ultraviolet plumage ornamentation affects social mate choice and sperm competition in bluethroats (Aves: Luscinia s. svecica): a field experiment

The blue throat feathers of male bluethroats (Luscinia s. svecica) show a reflectance peak in the ultraviolet (UV) waveband (320 to 400 nm). The throat is actively displayed during courtship, suggesting a role for sexual selection on an ultraviolet signal. Indeed, a recent aviary experiment demonstrated that females discriminated against males with artificially reduced UV reflectance (Andersson and Amundsen 1997). Here, we report the results of a similar experimental manipulation applied on free–ranging males. UV–reduced (UVR) males had a lower success in attracting mates, as judged from a significantly later start of egg laying, compared with control (C) males. UVR males also spent significantly less time advertising for additional mates when their own mate was fertile, and they had a lower success in achieving extra–pair fertilizations. Furthermore, UVR males tended to guard their mates more closely and lose more paternity in their own brood than C males did. We conclude that the treatment affected both social and extra–pair mate choice. This is the first experimental evidence that UV signalling influences male mating success in free–ranging birds.

Plumage colour in nestling blue tits: sexual dichromatism, condition dependence and genetic effects

Sexual–selection theory assumes that there are costs associated with ornamental plumage coloration. While pigment–based ornaments have repeatedly been shown to be condition dependent, this has been more difficult to demonstrate for structural colours. We present evidence for condition dependence of both types of plumage colour in nestling blue tits (Parus caeruleus). Using reflectance spectrometry, we show that blue tit nestlings are sexually dichromatic, with males having more chromatic (more saturated) and ultraviolet (UV)–shifted tail coloration and more chromatic yellow breast coloration. The sexual dimorphism in nestling tail coloration is qualitatively similar to that of chick–feeding adults from the same population. By contrast, the breast plumage of adult birds is not sexually dichromatic in terms of chroma. In nestlings, the chroma of both tail and breast feathers is positively associated with condition (body mass on day 14). The UV/blue hue of the tail feathers is influenced by paternally inherited genes, as indicated by a maternal half–sibling comparison. We conclude that the expression of both carotenoid–based and structural coloration seems to be condition dependent in blue tit nestlings, and that there are additional genetic effects on the hue of the UV/blue tail feathers. The signalling or other functions of sexual dichromatism in nestlings remain obscure. Our study shows that nestling blue tits are suitable model organisms for the study of ontogenetic costs and heritability of both carotenoid-based and structural colour in birds.

Agonistic carotenoid signalling in male red-collared widowbirds: aggression related to the colour signal of both the territory owner and model intruder

Carotenoid colour displays are widely assumed to be honest indicators of individual health or quality, primarily in mate attraction. Here we show that sexually dimorphic carotenoid ornamentation functions as an agonistic signal in male red-collared widowbirds, Euplectes ardens. Mounted male models differing (within natural limits) in the intensity of carotenoid signalling were presented to wild resident males as simulated intruders, perched or made to ‘fly’ across the territory with the elongated tail folded or keeled. Perched mounts were generally ignored, and stronger aggression towards ‘flying’ models with a keeled tail (i.e. as in courtship display) than a folded tail suggests the tail display is used to assess the intention of intruding males. Territory owners were less aggressive towards models with intense collar display, suggesting that carotenoid coloration functions as a badge of status in this species. The level of aggressive response was also related to the resident’s own badge in that males with larger, redder collars responded more aggressively to the models. In addition, males with a larger collar signal defended larger territories and spent less time in territory defence. Apart from the collar size and ‘redness’, no other morphological variable predicted the aggressive response of territorial males. Given the previously demonstrated insignificance of the collar in female mate choice, we suggest that the nuptial carotenoid coloration is an honest signal of dominance or fighting ability, sexually selected through male contest competition over territories. uf6d9 2001 The Association for the Study of Animal Behaviour

SEXUAL SELECTION OF MULTIPLE HANDICAPS IN THE RED‐COLLARED WIDOWBIRD: FEMALE CHOICE OF TAIL LENGTH BUT NOT CAROTENOID DISPLAY

Abstract.— Although sexual selection through female choice explains exaggerated male ornaments in many species, the evolution of the multicomponent nature of most sexual displays remains poorly understood. Theoretical models suggest that handicap signaling should converge on a single most informative quality indicator, whereas additional signals are more likely to be arbitrary Fisherian traits, amplifiers, or exploitations of receiver psychology. Male nuptial plumage in the highly polygynous red‐collared widowbird (Euplectes ardens) comprises two of the commonly advocated quality advertisements (handicaps) in birds: a long graduated tail and red carotenoid coloration. Here we use multivariate selection analysis to investigate female choice in relation to male tail length, color (reflectance) of the collar, other aspects of morphology, ectoparasite load, display rate, and territory quality. The order and total number of active nests obtained are used as measures of male reproductive success. We demonstrate a strong female preference and net sexual selection for long tails, but marginal or no effects of color, morphology, or territory quality. Tail length explained 47% of male reproductive success, an unusually strong fitness effect of natural ornament variation. Fluctuating tail asymmetry was unrelated to tail length, and had no impact on mating success. For the red collar, there was negative net selection on collar area, presumably via its negative relationship with tail length. None of the color variables (hue, chroma, and brightness) had significant selection differentials, but a partial effect (selection gradient) of chroma might represent a color preference when tail length is controlled for. We suggest that the red collar functions in male agonistic interactions, which has been strongly supported by subsequent work. Thus, female choice targets only one handicap, extreme tail elongation, disregarding or even selecting against the carotenoid display. We discuss whether long tails might be better indicators of genetic quality than carotenoid pigmentation. As regards the evolution of multiple ornaments, we propose that multiple handicap signaling is stable not because of multiple messages but because of multiple receivers, in this case females and males.

Correlations between ultraviolet coloration, overwinter survival and offspring sex ratio in the blue tit

We studied the correlations between offspring sex ratio, UV coloration and overwinter survival in a population of blue tits, breeding in Gotland, Sweden, over three consecutive breeding seasons. In 2 of 3u2003years, we found that females paired to males with relatively brighter UV‐coloration produced a greater proportion of sons in their broods, and that this effect was significant with all 3u2003years combined, despite a significant year by male UV interaction. In addition, we found other correlates of sex ratio (breeding time, female age and clutch size) in some, but not all years, and some of these showed significantly different relationships with sex ratio between years. In both years for which data were available, there were indications that males with relatively brighter UV coloration, and that paired with females that produced male‐biased clutches, were more likely to survive to the next year. In addition, we also found that in both males and females, individuals produced similar sex ratios in consecutive years. Because correlations with the sex ratio may be expected to be weak, variation in results between years within the same population may be explained by low statistical power or genuine biological differences. Our results suggest that conclusions about sex ratio variation in birds should be based on multiple years. The correlations that we found in some years of this study are consistent with models of adaptive sex ratio adjustment in response to mate quality. However, careful experimental work is required to provide tests of the assumptions of these models, and should be a priority for future work.

Female preference for long tails in lekking Jackson's widowbirds: experimental evidence

Abstract Jacksons widowbird, Euplectes jacksoni , is a lek-breeding weaverbird (Ploceidae) of East African grassland. Each male displays on his own display court: a cleaned circular track around a grass tuft. The male performs a jump display that attracts females, and a ground display directed at visiting females. Breeding males are black with a tail 20 cm long, which is conspicuously exhibited during display. Females and non-breeding males are streaky brown with a short tail. Tail length was the single most important predictor of male mating success in a previous study, suggesting that the tail is a cue for female mate choice. In the present experiment, males with shortened tails received fewer visits by females than control males, in 11 of 13 test pairs of neighbouring males. The rate of visits by females was correlated with male copulatory success. Display rate and lek attendance were not affected by tail manipulations. The results suggest that female Jacksons widowbirds prefer males with long tails. The experimentally produced difference in tail length roughly spans the standard deviation of a large sample of displaying males. This implies that cue assessment by females is accurate enough to impose sexual selection on male tail length. A previously found positive relationship between tail length and an index of body condition was strengthened by the increased sample size. The results are discussed in relation to current sexual selection theory.

Paternity analysis reveals opposing selection pressures on crown coloration in the blue tit (Parus caeruleus)

In socially monogamous species, extra–pair paternity can increase the variance in reproductive success and thereby the potential for sexual selection on male ornaments. We studied whether male secondary sexual ornaments are selected through within– and/or extra–pair reproductive success in the blue tit (Parus caeruleus). Male blue tits display a bright blue crown plumage, which reflects substantially in the ultraviolet (UV) and previously has been indicated to be an important sexual signal. We show that males with a more UV–shifted crown hue were less cuckolded, which probably resulted from female preference for more ornamented mates. By contrast, however, older males and males with a less UV–shifted hue sired more extra–pair young. This probably did not reflect direct female preference, since cuckolders were not less UV–ornamented than the males they cuckolded. Alternatively, a trade–off between UV ornamentation and other traits that enhance extra–pair success could explain this pattern. Our results might reflect two alternative male mating tactics, where more UV–ornamented males maximize within–pair success and less UV–ornamented males maximize extra–pair success. Since crown colour was selected in opposite directions by within–pair and extra–pair paternity, directional selection through extra–pair matings seemed weak, at least in this population and breeding season. Reduced intensity of sexual selection due to alternative mating tactics constitutes a potential mechanism maintaining additive genetic variance of male ornaments.