Graduated tail of birds is believed to be a significant secondary sexual trait, and takes an important role in sexual selection. By means of secondary sexual trait, most species of birds handicap the existence of male cheaters, which arise in competition among males and mate selection. However, some species of Cisticolidae, Passerformes show a converse change, in which graduated tail of breeding plumage is shorter than that of winter plumage. It means that their sexual traits are lost in breeding season. It seems that this pattern is conflicted with Sexual Selection Theory, and may reveal a new survival strategy.
In order to explain how this peculiar pattern influences breeding strategy,we studied on yellow-bellied prinia, Prinia flaviventris at Nansha District, Guangzhou for about one year. Yellow-bellied prinia is monomorphic and monogamous. Both males and females have a graduated tail which is even longer than their body. It moults twice every year, including complete moult after breeding from September to December, and partial moult before breeding from March to April. Its tail is replaced in both spring moult and autumn moult, and more importantly, tail length of breeding plumage is shorter than tail length of winter plumage.
We used mate-choice cage to test whether females handicapped male-cheaters by stable mate relationship on yellow-bellied prinia. Our experiment consisted of 21 trials, and each trial lasted about 1 hour. In each trial, both experimental males had no significant differences of body measurements, besides the length of tail. The mate-choice cage was isolated with black cloth to minimize human disturbance, and we recorded all the experiments by Sony DCR-VX2000E video camera. In order to minimize influence of stress reaction, we ignored the beginning and ending of every experiment and only process from 16^th min to 45^th min(totally 1800s)of each videotape. One-way ANOVA was applied to compare different options of females to males and sexual behavior between male groups. All statistics and analysis was conducted using software SPSS 17.0.
In mate-choice cage tests, the result shows that selecting times of females to both mate group and comparison group were no significant difference, but females spent more time in front of males from mate group. As breeding time went on, this trend was becoming more obvious, that the mate relationship were more and more stable. With respect to behaviors of males, jumping times of mate group was significantly greater than those of comparison group. The times of overawing of mate group were also significantly greater than those of comparison group. Hence, we summarized that females preferred their mate males, and this preference would be more intensity with the breeding time. Mate males would do more sexual display and threaten display than extra-pair males.
According to this experiment, we considered that mate males moulted after mating, consequently, they would reduce additional investment and increase flight flexibility. Besides, both males and females moulted in breeding season, females also face the threat of cheaters. In the process, mate males will protect their consorts.Thus we summarized that the high survival cost for breeding, better aerodynamic effect of males, competition among females, and ownership effect of mate males might be principal causes of this result.
We concluded that stable mate relation in species whose sexual traits were lost in breeding season could handicap the existence of male-cheaters. We conjectured that breeding investment and competition between females might be the main reasons for stale mate relationship.