Carlos Navarro

Sexually Selected Egg Coloration in Spotless Starlings

It has been recently proposed that the blue‐green coloration in eggs of many avian species may constitute a sexually selected female signal. Blue‐green color intensity would reflect the physiological condition of females, and hence it might also affect the allocation of male parental care. In this study, we use three different experimental approaches to explore the importance of sexual selection on blue‐green egg coloration of spotless starling (Sturnus unicolor) eggs. First, experimental deterioration of female body condition (by means of wing feather removal) negatively affected the intensity of blue‐green egg coloration. Second, blue‐green color intensity of artificial model eggs had a significant positive influence on paternal feeding effort. Finally, we found a negative relationship between the effect of experimental food supply on nestling immunocompetence and the intensity of blue‐green coloration of eggs, suggesting that egg color predicts nutritional conditions that nestlings will experience during development. All these results taken together strongly support a role of sexual selection in the blue‐green coloration of spotless starling eggs.

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.

Digit ratios (2D:4D), secondary sexual characters and cell-mediated immunity in house sparrows Passer domesticus

Homeobox genes regulate development of digits, and it has been suggested that the ratio of length of second to length of fourth digit reflects such genetic effects in a sex-specific manner. We show that digit ratios in the sexually dichromatic house sparrow Passer domesticus differ between sexes, with males having higher ratios than females, and that individuals produce consistent ratios on the two feet. If Homeobox or other genes had pleiotropic effects on development of digits, behavior, and physiology of males and females, we would expect secondary sexual characters and immunity to be related to digit ratio in a sex-specific manner. The size of the visible part of the black badge in February (a secondary sexual character), but not total badge size, was positively correlated with digit ratios, suggesting that males with more male-like digit ratios had larger visible badges. Because of sex-specific effects of development on secondary sexual characters and immunity, we predicted sex-specific differences in immune response to be related to digit ratio. House sparrows with large digit ratios had weaker T cell-mediated immune response than individuals with small digit ratios, particularly in females, implying that females with more male-like digit ratios had weak immune responses. These findings are consistent with the hypothesis that early development, as reflected by digit ratios, and genetics affect the expression of adult characters that are supposedly strongly contributing to fitness.

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.

Male spotless starlings adjust feeding effort based on egg spots revealing ectoparasite load

Parents may vary their parental behaviour and investment in reproduction in response to parasite-induced changes in the fitness prospects of their offspring. Thus, parents may use the physical condition of their offspring, or any other trait related to parasite load, to adjust parental effort. The immaculate eggs of the spotless starling, Sturnus unicolor , often become densely spotted owing to the activity of the ectoparasite carnid fly Carnus hemapterus . Spot density anticipates the intensity of fly infestation suffered by nestlings and, therefore, may serve as a cue for parents to adjust reproductive investment. By cleaning spots produced by C. hemapterus on eggs of spotless starlings, we manipulated the parasites traces revealing its presence in broods of starlings, without modifying the level of infestation, to test whether parents use these signals to adjust reproductive effort. We found support for the hypothetical negative effect of Carnus flies since nestlings raised in nests with a higher fly load had lower body mass. The experimental egg cleaning during incubation did not change the intensity of carnid fly infestation during nestling development. However, it had a significant positive influence on paternal but not maternal effort. Our experimental results support the idea that spotless starling males adjust their effort in response to their perception of the fitness prospects of their nestlings as indirectly estimated by traces of parasites on the eggshells. As far as we know, this is the first evidence of the use of parasite traces to infer risk of parasitic infestation by animal hosts.

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.

Experimental feeding affects the relationship between hematocrit and body mass in Spotless Starling (Sturnus unicolor) nestlings

Hematocrit, the proportion of blood volume occupied by packed red blood cells, is frequently used as an estimate of phenotypic condition. Some studies in birds, however, suggest that hematocrit might not always be a good estimate of condition. We tested the reliability of hematocrit as an estimate of condition by investigating the relationship between hematocrit and two other estimates of phenotypic condition (body mass controlled for body size and T-cell-mediated immune response) in nestlings of the Spotless Starling (Sturnus unicolor) under different environmental conditions. Half of each brood received an experimental food supplement while the other half was kept as a control. Hematocrit was positively related to relative body mass only in control nestlings whereas the relationship between hematocrit and cellular immune response was far from significant in both groups of nestlings. As expected, experimental food supply weakened the relationship between hematocrit and relative body mass, but this effect was not mediated by a decrease in the variation of phenotypic condition among nestlings. Instead, the effect of food supplementation was condition-dependent, reducing hematocrit in heavier than average nestlings, and increasing hematocrit in lighter than average nestlings. These results suggest that hematocrit might not be a reliable estimate of phenotypic condition under certain nutritional circumstances.

Do spotless starlings place feathers at their nests by ultraviolet color

A considerable number of bird species carry feathers to their nests. Feathers’ presence in the nests has traditionally been explained by their insulating properties. Recently, however, it has been suggested that feathers carried to the nests by females of the spotted starling (Sturnus unicolor L.) could have an ornamental function based on their ultraviolet (300–400xa0nm) and human-visible longer wavelength (400–700xa0nm) coloration. In our population, 95.7% of feathers found inside next-boxes occupied by nesting starlings were rock dove fly feathers. Of these feathers, 82.7% were naturally positioned with their reverse side oriented toward the entrance hole and 42.4% of all found feathers were situated within the nest-cup. Here we experimentally assess the signaling function of ultraviolet coloration of feathers in nests of spotless starlings by providing nests with a number of pigeon flight feathers that were respectively treated on their obverse, reverse, both, or neither side with a UV blocker. Starlings placed 42.5% of the experimental feathers in the nest-cup irrespective of the UV block treatment. Orientation of feathers toward the entrance hole was not related with their ultraviolet radiation. However, feathers placed within the nest-cup were more likely found with their reverse side oriented toward the entrance hole confirming our correlative findings. These results suggest a minor role of ultraviolet coloration on feather location by spotless starlings.

Erratum to Beak colour reflects circulating carotenoid and vitamin A levels in spotless starlings (Sturnus unicolor) (Behav Ecol Sociobiol, 10.1007/s00265-010-0920-5)

The original version of this article unfortunately contained numerous spelling and grammatical mistakes, but also the following factual errors that we wish to correct here: 1) The surname of one of the authors was misspelled. Instead “Kevin J Mcgraw” should read “Kevin J McGraw”. 2) The journal for one of the citations in the reference list was incorrectly given. Here is the corrected entry: “Saks L, McGraw K, Horak P (2003) How feather colour reflects its carotenoid content. Funct Ecol 17:555–561”. 3) In the legend for Tables 2 and 3, “total carotenoid plasma level (TCPL)” should read “total plasma carotenoid level (TPCL)”. Also in Table 2, “Date2” should be “Date”. 4) Throughout the Methods and Results sections, periods were used to separate the degrees of freedom for the numerator and denominator of F statistics; instead, these should be separated by commas. 5) In the 6th line of the 2nd paragraph of the Discussion section, “plasma-serum ratio” should be replaced with “plasma/red blood cells ratio”.