Juan J. Soler

Rejection of parasitic eggs in relation to egg appearance in magpies

The coevolutionary process between avian brood parasites and their hosts predicts that low intraclutch variation in egg colour appearance favours egg discrimination of parasite eggs by hosts. Low intraclutch variation would also result in high interclutch variation, which would increase the difficulty of evolution of mimicry by the cuckoo, because many host colour patterns might coexist in the same host population. We explored this possibility using an experimental approach in the common magpie, Pica pica, and great spotted cuckoo, Clamator glandarius, system. We artificially parasitized magpie nests with great spotted

Dark Nests and Conspicuousness in Color Patterns of Nestlings of Altricial Birds

Nests of altricial birds exhibit variable spectral properties that may affect the efficacy (conspicuousness) of the colored begging traits that a nestling displays to its parents. Here we explored whether selection for efficient perception has favored the evolution of nestling color designs that maximizes nestling detectability in variable light environments. Visual models were used to estimate how parents perceive the coloration of mouths, flanges, heads, and breasts of nestlings within their nest in 21 species of European birds. We show that the largest chromatic and achromatic contrasts against the nest background appeared for nestling mouths and flanges, respectively. Nestlings of open‐nesting species showed a larger general achromatic contrast with the nest than did nestlings of hole‐nesting species. However, nestlings of hole nesters showed a more evident achromatic contrast between flanges and other traits than did nestlings of open nesters. In addition, species with larger clutch sizes showed larger general achromatic contrasts with the nest. Gaping traits of open‐nesting species contrasting with the nest background were better perceived under rich light regimes than under poor ones. These findings are consistent with a scenario in which selection for nestling detectability in dark environments has favored the evolution of particular achromatic components of gape coloration but also nestling traits that enhance signal efficacy by maximizing color contrasts within a nestling.

Dark nests and egg colour in birds: a possible functional role of ultraviolet reflectance in egg detectability

Owing to the conspicuousness of ultraviolet (UV) colour in dark environments, natural selection might have selected UV egg coloration because it would enhance egg detectability by parents in murky nests. Here, we tested this hypothesis by using comparative and experimental approaches. First, we studied variation in egg coloration of 98 species of European passerines measured using UV–visible reflectance spectrometry (300–700u200anm) in relation to nesting habits. Analyses based on raw data and controlling for phylogenetic distances both at the species and the family levels revealed that hole-nester species produced eggs with higher UV reflectance than those nesting in open habitats. The experimental approach consisted of the manipulation of UV reflectance of the experimental eggs introduced outside the nest-cup of the hole-nester spotless starling Sturnus unicolor and the study of the retrieval of these eggs. Ultraviolet-reflecting eggs (controls) were more frequently retrieved to the nest-cup than non-reflecting (–UV) eggs. These results were not due to ‘–UV’ eggs being recognized by starlings as parasitic because when a parasitic egg is detected, starlings removed it from the nest-box. Therefore, these results are consistent with the hypothesis that UV egg colours are designed to provide highly detectable targets for parent birds in dark nest environments.

Nest-building activity and laying date influence female reproductive investment in magpies: an experimental study

Nest size or nest-building activity has recently been hypothesized to be a postmating sexually selected signal in monogamous birds: females may assess a male’s parental quality and willingness to invest in reproduction by his participation in nest building. Females may thus adjust their reproductive effort (i.e. clutch size) not only to their own abilities but also to those of their mates. We investigated whether female magpies, Pica pica, use nest-building activity rather than nest size to adjust their reproductive effort during replacement breeding attempts. After we removed their first clutch, high-quality pairs that built a large nest for the first clutch were more capable of building a replacement nest and females adjusted their clutch size in relation to the time it took to build the nest rather than nest size. We also found support for the hypothesized trade-off between clutch size and egg size in magpies. In replacement clutches females decreased clutch size and increased egg volume, thereby probably improving the survival probability of their offspring in less favourable conditions. uf6d9 2002 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour

CUCKOO PARASITISM AND PRODUCTIVITY IN DIFFERENT MAGPIE SUBPOPULATIONS PREDICT FREQUENCIES OF THE 457bp ALLELE: A MOSAIC OF COEVOLUTION AT A SMALL GEOGRAPHIC SCALE

Abstract The level of defense against great spotted cuckoo (Clamator glandarius) parasitism in different European populations of magpie (Pica pica) depends on selection pressures due to parasitism and gene flow between populations, which suggests the existence of coevolutionary hot spots within a European metapopulation. A mosaic of coevolution is theoretically possible at small geographical scales and with strong gene flow, because, among other reasons, plots may differ in productivity (i.e., reproductive success of hosts in the absence of parasitism) and defensive genotypes theoretically should be more common in plots of high productivity. Here, we tested this prediction by exploring the relationship between parasitism rate, level of defense against parasitism (estimated as both rejection rate and the frequency of the 457bp microsatellite allele associated with foreign egg rejection in magpies), and some variables related to the productivity (average laying date, clutch size, and number of hatchlings per nest) of magpies breeding in different subpopulations. We found that both estimates of defensive ability (egg rejection rate and frequency of the 457bp allele) covaried significantly with between-plot differences in probability of parasitism, laying date, and number of hatchlings per nest. Moreover, the parasitism rate was larger in more productive plots. These results confirm the existence of a mosaic of coevolution at a very local geographical scale, and the association between laying date and number of hatchlings with variables related to defensive ability and the selection pressure arising from parasitism supports the prediction of coevolutionary gradients in relation to host productivity.

Beak colour reflects circulating carotenoid and vitamin A levels in spotless starlings (Sturnus unicolor)

Many colourful sexually selected signals in animals are carotenoid-dependent and, because carotenoids function as antiradicals and immunostimulating molecules, carotenoid-dependent signals may honestly reflect the health state of individuals. Some others nutrients like vitamin A may also enhance health and colouration, but these have rarely been tested alongside carotenoids in colourful birds. Here, we examined whether beak colour of the spotless starling (Sturnus unicolor) reflected circulating levels of carotenoids and/or vitamin A (retinol). Spotless starlings are polygynous, sexually dimorphic birds (i.e. length of chest feathers). The tip of the beaks of male and female spotless starlings is more intensely coloured at the beginning of the breeding season and becomes dull after mating, which may suggest a sexual function. We found that females have a more intensely coloured beak and higher plasma carotenoid concentration than males during mating, and, despite the finding that carotenoid and vitamin A levels were not significantly related; colour intensity was positively correlated with plasma concentration of carotenoids and vitamin A in both sexes. However, adult beak coloration was not associated with carotenoid and vitamin A concentrations after nestlings were hatched. Therefore, beak colouration of spotless starlings provides information about circulating levels of carotenoids and vitamins during the mating season and may potentially function as a reliable signal of physiological status in the context of sexual selection.

Is the relation between colour and immune response mediated by nutritional condition in spotless starling nestlings

The hypothesis that nestling coloration is important for parenteoffspring communication, because it influences parental feeding decisions, has received strong experimental support. In European starlings, Sturnus vulgaris, and Alpine swifts, Apus melba, manipulation of ultraviolet reflectance of nestlings’ mouth and skin affected the amount of food parents provided, and skin brightness of starling nestlings predicted their T-cell-mediated immune response. Therefore, a link between nestling coloration and immunity, mediated by parental effort, was suggested. We explored this hypothesis by experimentally feeding some spotless starling, Sturnus unicolor, nestlings while leaving others in the same nest as a control. First, we found a significant effect of food supplementation on nestlings’ immune response, which is a requirement for the hypothesis. Second, we confirmed in spotless starlings the association between skin brightness and ability to raise an immune response. However, this correlation disappeared when we controlled for between-nest variation. These results suggest that parental feeding preference is not the only factor explaining nestling immunity, and that covariation between mean brood nestling coloration and parental quality, and/or intrinsic (i.e. genetic) quality of nestlings, may explain the association between immunity and coloration of nestlings. Finally, within-nest variation in nestling coloration partially explained immune responses because food supplementation had more effect on nestlings with brighter skin. We discuss these results as possible evidence of nestling coloration partially reflecting intrinsic characteristics that affect both ability to produce efficient immune responses and parental feeding preferences.

Sexual selection based on egg colour: physiological models and egg discrimination experiments in a cavity-nesting bird

It has been proposed that the blue-green bird egg colourations of many avian species may constitute a sexually selected female signal that males can use to modulate their parental investment. A fundamental untested assumption for the validation of this hypothesis is that males can accurately assess differences in the colour of eggs. A recent review suggests that this could be particularly problematic when egg clutches were located within a dimly lit nest cavity, due to limitations of the visual system in low light conditions. Here, we first used a photoreceptor noise-limited model of colour discrimination ability that accounts for visual performance under low light conditions to study whether a typical cavity-nesting passerine, the spotless starling Sturnus unicolor, can discriminate their eggs under the ambient illumination in their nest-holes. Secondly, we tested the validity of model predictions with behavioural data collected in two egg discrimination experiments performed in this species. Estimated egg detectability depended entirely on model assumptions about visual limitations linked to light intensity. Starlings would not be able to discriminate egg differences in their nests if the model was based on the assumption that light intensity limited detectability, whereas they could potentially perceive as different many possible pairwise clutch comparisons if the model assumption was that light intensity did not limit detectability. Results of behavioural experiments fitted the prediction of the visual model where light intensity did not limit detectability. Our results suggest that photoreceptor noise-limited colour models based on stimulation of single photoreceptors cannot, at present, be used to predict egg discrimination ability in spotless starlings under low light conditions. Future studies aiming to test egg discrimination constraints in the frame of the sexual selection hypothesis should therefore combine both modelling and behavioural experiments to determine which are the components of the models that produce the mismatch with the behavioural conditions.

Geographic distribution of suitable hosts explains the evolution of specialized gentes in the European cuckoo Cuculus canorus

BackgroundSeveral types of selective forces can act to promote parasite specialization. Parasites might specialize on some suitable hosts at the cost of decreasing effectiveness when exploiting other species of hosts, and specialization can be more easily selected for in hosts that the parasites will easily find. Thus demographic characteristics of suitable hosts such as population density and its spatial consistency could be key factors predicting probability of parasite specialization and speciation. Here, we explore this hypothesis by studying the relationship between occurence of specialized races of the European cuckoo (Cuculus canorus) (i.e. gentes) and mean and coefficient of variation in population density estimated for 12 different European regions.ResultsThe results were in accordance with the hypothesis because specialized cuckoo egg morphs were more common in suitable hosts with high population density and low variation in population density at the level of host species or genera.ConclusionWe have presented evidence suggesting that population density and homogeneity of geographic distribution of hosts explain, at least partly, the evolution of specialized egg-morphs of the European cuckoo. These results are consistent with the hypothesis that resource (i.e., host) predictability explains the evolution of host races and species of parasites.

Spotless starlings rely on public information while visiting conspecific nests: an experiment

Animals may acquire information on potential breeding sites by prospecting, which allows future optimal selection of breeding territories. The reproductive success of conspecifics, as public information (PI), has been proposed as one of the cues that prospectors could gather and then use for future reproductive decisions. We experimentally decreased brood size in spotless starling nests to investigate whether this species gathers PI while visiting conspecific nests. We expected visiting frequency to decline with the experimental decrease in brood size because visitors are expected to prefer sites with high reproductive success and to spend more time at those sites to gain familiarity. Furthermore, the effect of the experimental manipulation was recorded at three different stages of the nestling period to establish the importance of the reliability of PI. Brood size decreased in direct relation to the manipulation and, consequently, parental feeding rates decreased too. Visiting frequency of starlings to conspecific nests was affected by the interaction between the experimental manipulation and the stage in the nestling period: visiting frequency increased from decreased to control nests at the end of the nestling period, not so strongly in the middle of that period and it was not affected by the manipulation at the beginning of it. This variation in visiting frequency seemed to be better explained by brood size than by parental provisioning rate. These results may be interpreted as spotless starlings visiting conspecific nests to gather PI, which seems to increase its informative value when its reliability does.