Johnsen Arild, Lifjeld Jan T
Max Planck Research Centre for Ornithology, P.O. Box 1564, 82305 Starnberg (Seewiesen), Germany.
Oecologia. 2003 Aug;136(3):476-83. doi: 10.1007/s00442-003-1286-4. Epub 2003 May 29.
The mating system of the bluethroat (Luscinia s. svecica) involves a high level of sperm competition, and consequently a high frequency of extra-pair paternity (EPP). There is considerable variation in the frequency of EPP. Over the course of ten study years in a population in Norway, the frequency has fluctuated between 7% and 33% of young, and 8% and 76% of broods. In this paper, we address the issue of whether ecological factors can explain some of the variation in EPP between years and broods. Factors include breeding density, breeding synchrony and two meteorological variables (ambient temperature and precipitation) during the period of peak female fertility. There were no significant relationships between annual averages of the four variables and the annual level of EPP, but the statistical power of these tests was low, due to a restricted sample (n=10 years). Focussing on individual broods, none of the factors had significant effects when including all broods in the analyses (including those with zero EPP). When limiting the analyses to broods with one or more extra-pair offspring, morning temperature had a significant effect on the frequency of EPP. When the temperature was low during the peak of the fertile period, EPP occurred less frequently. Male extra-pair behaviour was not generally constrained by synchrony between the fertile periods of the social mate and the extra-pair mate, but for a subgroup of males ("unattractive males") there appeared to be a trade-off between mate guarding and pursuing extra-pair copulations. Our results indicate that ecological conditions have no influence on whether or not a brood will contain extra-pair offspring, but when EPP does occur, its frequency is influenced by air temperature. Possibly, there is a causal relationship between weather conditions and the frequency of extra-pair encounters in this species. The breeding season is initiated at a phenologically early stage, when large parts of the territories are still covered with snow, and cold weather conditions may force the birds to devote most of their time to maintenance and thereby constrain sexual activities.
蓝喉歌鸲（Luscinia s. svecica）的交配系统涉及高水平的精子竞争，因此婚外父权（EPP）的频率较高。EPP的频率存在相当大的差异。在挪威一个种群的十年研究期间，幼鸟的EPP频率在7%至33%之间波动，窝雏的EPP频率在8%至76%之间波动。在本文中，我们探讨生态因素是否能解释年份和窝雏之间EPP的一些差异。这些因素包括繁殖密度、繁殖同步性以及雌性生育高峰期的两个气象变量（环境温度和降水量）。这四个变量的年平均值与EPP的年度水平之间没有显著关系，但由于样本有限（n = 10年），这些测试的统计效力较低。关注单个窝雏，在分析中纳入所有窝雏（包括EPP为零的窝雏）时，没有一个因素有显著影响。当将分析限制在有一个或多个婚外后代的窝雏时，早晨温度对EPP频率有显著影响。在生育高峰期温度较低时，EPP发生的频率较低。雄性的婚外行为通常不受社会配偶和婚外配偶生育期同步性的限制，但对于一部分雄性（“缺乏吸引力的雄性”）来说，在配偶守卫和寻求婚外交配之间似乎存在权衡。我们的结果表明，生态条件对一窝是否会有婚外后代没有影响，但当EPP确实发生时，其频率受气温影响。可能在这个物种中，天气条件与婚外相遇频率之间存在因果关系。繁殖季节在物候学上较早开始，此时大部分领地仍被积雪覆盖，寒冷的天气条件可能迫使鸟类将大部分时间用于维持生存，从而限制了性活动。
