The Seasonal Decline in Avian Clutch Size: Parental Condition or Environmental Deterioration?

鸟类学中最有力的概括之一是，在季节较晚产卵的鸟类每窝产的蛋往往更少。对于这种模式的原因存在相当大的分歧。一种观点认为，季节性下降是雌性条件变化的非适应性结果，雌性条件变化对产卵时间和窝卵数量都有重要影响。另一派认为，季节性的窝蛋数量减少是对产卵雌性食物供应变化的适应性反应，或者是抚养后代的前景季节性下降。我们建议用三种方法来区分这些可能性。其中一个实验将通过降低觅食能力和产卵前雌性的状况来测试食物供应对产卵雌性的影响，并寻找产卵时间和窝数的预测变化。第二项实验将改变养鸡的时间。如果条件对雏鸡的饲养成功有压倒性的影响，那么操纵将对群体间的后代生产没有影响。另一方面，如果繁殖环境起主导作用，则不同群体的后代产量会有所不同。统计测试将获取工况、离合器尺寸和产蛋日期的数据，并通过部分回归分析，在条件控制后，寻找产蛋日期对离合器尺寸的显着影响。在所有这些测试中，将借助新开发的无损技术以脂肪和蛋白质储存量来测量状况。这些结果不仅引起对身体状况和鸟类历史之间相互作用感兴趣的生理生态学家的兴趣，而且引起对生活史变异感兴趣的种群生物学家的兴趣。

One of the strongest generalizations in ornithology is that birds that lay later in the season tend to lay fewer eggs per clutch. There is considerable disagreement over the causes of this pattern. One school maintains that the seasonal decline is a nonadaptive consequences of variation in female condition, which has an overriding effect both on timing of laying and clutch size. The other school sees seasonal clutch size declines as an adaptive response to variation in food availability for the laying female or seasonally declining prospects for rearing offspring. It is proposed to distinguish between these possibilities in three ways. One experiment will test for the effect of food supply for the laying female by decreasing the foraging profitability and the condition of the female prior to laying and looking for predicted shifts in timing of laying and clutch size. A second experiment will alter the timing of chick-rearing. If condition has the overwhelming effect on chick-rearing success, then the manipulation will have no effect on offspring production across groups. If, on the other hand, the breeding environment has a predominant effect, offspring production will vary across groups. A statistical test will take data on condition, clutch size, and laying date, and, through partial regression analysis, look for a significant effect of laying date on clutch size once condition is controlled for. In all these tests, condition will be measured in terms of fat and protein stores with the aid of newly developed nondestructive techniques. These results should be of interest not only to physiological ecologists interested in the interaction between body condition and avian histories, but also to population biologists interested in life history variation.