In the paper by Cuervo et al, explain a) why it was important, from the perspective of being able to demonstrate the influence of sexual selection, that the authors followed up on their original (1996) study; and b) why, if they were attempting to demonstrate that sexual selection was acting to lengthen male tail length, they studied females and not males.
Paper title: Experimental manipulation of tail length in female barn swallows (Hirundo rustica) affects their future reproductive success
Sexual selection(any phenotypic variation nonrandomly related to variation in mating success) often explians the presence of apparently non adaptive traits in many animal species.Secondary sexual characters might be determinental in terms of survival,but if they confet mating advantages,they could pass to the next generation.Typical examples of sexually selected traits are the exaggarated long male tails of many birds Bird species with long tails and intensively studied is the barn swallow(Hirundo rustica),but in this case,the function of long tail streamers has been the subject of a long debate.
Most studies trying to identify the function of external tail feathers in the barn swallow have focused on males,much less attention has been paid to females.In barn swallows,females also have considerably long tails,longer than those of juveniles of the two sexes,but significantly shorter than males.There is not agreement if tail length in female barn swallows is the optimum according to natural selection or if it has been elongated beyond that optimum by sexual selection
An experimental study in different population did not find any evidence for tail length in females being an ornnament assumed that tails in female barn swalows were longer than the optimum according to natural selection,because females had longer tails than juveniles.Tail elongation in females could have been a consequence of strong directional female preference for long tailed males if there is a strong genetic correlation between the character in the two sexes.Another possiility is that long tails also confer mating advantages to females.If this is the case ,long tails in females would reflect simple attractiveness.
Manipulated the length of external tail feathers,but they did not find any evidence that male mating preferences depended on female tail length,thus supporting the correlated response hypothesis for the existance of exaggerated long tails in feales.Moreover,they find that neither experimental treatment nor original female tail length previous to the treatment had the significant effect on the number of reproductive variables,start of laying,offspring provisioning,total no of eggs and total no of fledglings.
In attempt to understand the fuction of external tail feathers in female barn swallows,as well as the evolutionary forces that have driven their evolution,Cuervo et al,also calculated daily energy expenditure in the females with experimentally manipulated tail length.If tail length is optimal according to natural selection,any tail length modification will impair flight performance and will cause either a change in behaviour or an increase in energy expenditure.On the other hand,if tail feathers have been elongated by sexual selection beyond the optimum according to natural selection,experimental elongation of this tail will also impair flight efficiency.However,experimental shortening might reduce tails to a length closer to natural selection optimum,and flight performance would then improve.Cuervo et al.found no significant evidence for behavioral changes in the birds involved in the experiment.Surprisingly,experimental treatment had no significant consequences on energy expenditure,although both natural and sexual selection hypothesis predicted some effect of tail length manipulation on flight performance and on energtic costs.Probably assessing energy expenditure during a short period of time,was not the most appropriate method to detect costs of tail length manipulation.