The functional signi¢cance of elongated, narrow tips of the tail feathers of certain birds, so-called tail streamers, has recently been discussed from an aerodynamic point of view, and the e¡ects of sexual selection on such traits have been questioned. We review our long-term ¢eld studies using observational and experimental approaches to investigate natural and sexual selection in the barn swallow, Hirundo rustica, which has sexually size-dimorphic outermost tail feathers. Experimental manipulation of the length of the outermost tail feathers has demonstrated sexual selection advantages of tail elongation and disadvantages of tail shortening, with opposite e¡ects for natural selection in terms of foraging e¤ciency, haematocrit and survival. These ¢ndings are contrary to the prediction of a general deterioration from both shortening and elongation, if the tail trait was determined solely by its e¡ects on aerodynamic e¤ciency and £ight manoeuvrability. Patterns of sexual selection in manipulated birds conform with patterns in unmanipulated birds, and selection di¡erentials for di¡erent components of sexual selection in manipulated birds are strongly positively correlated with di¡erentials in unmanipulated birds. Age and sex di¡erences in tail length, and geographical patterns of sexual size dimorphism, are also consistent with sexual selection theory, but inconsistent with a purely natural selection advantage of long outermost tail feathers in male barn swallows.

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.

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