Here is a graph from the original paper with an abbreviated

Here is a graph from the original paper with an abbreviated figure legend: Fig 1: Mating success in male long-tailed widowbirds subjected to different tail treatments, a. Mean number of active nests per territory for the nine males of the four treatment categories before the experiment to. Number of new active nests in each territory after treatment of males. The numbers in the bars are the number of nests of each of the 9 males The height of the bar is the average of those numbers Differences between nests after and nests before treatment were compared using a modified Pittman randomization test Is mating random? Do the data appear to support the hypothesis? Support your answer Suggest a reason why females should prefer males with longer tails. In other words, what might females gain by choosing these males over males with shorter tails? You may have to do some reading to provide a good answer. If females prefer males with longer tails, why don\'t males evolve tails that are many feet long? A similar study was conducted on another species of widowbird that only has short tails. In this species females still preferred males with experimentally elongated tails although they had no experience with males with long tails. Suggest an explanation for this preference.

Solution

Long tail, an aesthetic feature of male birds is appreciated by female bird for mating. This reminds me about the sexual selection (survival of the sexiest/fittest rule of Darwin).
The long feather cost very little to bird for weight and energy but gives big advantage of looking bigger, stronger and healthier before mating counterparts and females prefers this features as an intrinsic nature. There are many debate against the survival disadvantage of long tail but I have seen birds escaping from cat’s mouth leaving their fancy tail in cat’s mouth. The tail regrows once detached from the bird.

In the given experiment the tail length was altered (shortened and lengthened) in widowbird. The mean number of new (after alteration) active nests in each territory was found significantly increased in experimentally elongated tailed birds and slightly decreased in case of short tailed birds in comparison to unaltered birds.
a)   From the result it is clear that the mating is not random but the males with added length in their tail got better mating opportunity and successfully produced more nests. So it supports the theory of sexual selection. The tallest bar in favour of long tailed mail proves that long tailed males had shown more sex appeal and more appealing for mating.
b)   The long tailed birds had greater sex appeal and looked bigger, vigorous, stronger and healthier which is the intrinsic drive for mate in female bird like in “tall dark and handsome” in human. Females expected healthy, bigger, vigorous and stronger male offspring with aesthetic value those will have better chance to have sex or find their mates in their future. Whereas small tailed birds looked like weak, unhealthy and underdeveloped.
c)   There is no doubt that female birds prefer bigger tailed mail mate and accordingly many mail birds evolved to have bigger tails but there is a limit of extra-long tail as it exerts metabolic, physiological, flight burden and it is difficult to survive with very long tail. So there is a trade-off between looking sexy at a reasonable cost of survival.
d)   Female have intrinsic nature of selecting healthy, bigger, vigorous and stronger male mate. It is in their genes. Due to evolutionary history of liking to long tail the female birds possess the gut feeling to select a bigger, healthier, vigorous and stronger male mate.