Proximate Causes of Parental Desertion

在许多种类的动物中，父母的照顾对于新生后代的生存是必不可少的，但有时在后代达到完全独立之前很久，父母就终止了这种照顾。造成这种遗弃的生态原因是多样的，但集中在最大化繁殖成功的进化原则上。换句话说，从长远来看，父母或许能够在事业已经失败时(例如，在干旱期间，如果粮食基础急剧恶化)削减投资，从而养育更多的孩子。最近对鸟类(白鹭和苍鹭)的实地研究发现了另一种情况，在这种情况下，父母似乎会抛弃已经孵化的幼崽。当雏鸟的数量从三只或四只下降到只有一只存活下来的雏鸟时，父母可能会放弃这一点，利用夏季繁殖季节的剩余时间来重新繁殖，从而可能产生更大的雏鸟。父母行为发生这种转变的信号大概是巢中雏鸟的数量。这一想法已经得到了两个物种的观测数据的支持，但还没有经过实验验证。本研究旨在利用一种非常常见的物种(牛鹭)进行一系列补充的野外实验，以填补这一空白。在第一个实验中，通过将新孵化的雏鸟转移到附近体型较小的雏鸟那里，它们将被人为地从三只雏鸟减少到只有一只，在那里它们很容易被接受。控制组将由幼崽组成，它们的幼崽得到相同数量的处理，但随后被送回自己的巢中。莫克博士预测，与对照组相比，父母会更频繁地抛弃单身子女。为了探索这种影响是否会随着繁殖季节的剩余数量减少而减少，第一个实验将在整个筑巢期间进行三次，预测较晚的单胞胎比季节早期的单胞胎更不可能被遗弃。如果是这样的话，父母将不得不同时考虑雏鸟的数量和季节。最后一个实验涉及到雏鸟本身的行为。在这个物种和相关物种中，雏鸟在不同的时间孵化，略大和较大的雏鸟会迫害它们的弟弟妹妹，甚至到了杀死他们的地步。如果成为独生子女是一种危险的情况(增加父母遗弃的风险)，那么当雏鸟的数量从三个减少到两个时，小鸡可能会降低与兄弟姐妹打架的比率。这将通过将三只雏鸟中最小的一只从持续观察的鸟巢中取出，然后在三天后更换它来进行测试。如果攻击性取决于筑巢的数量，那么当只有两只雏鸟出现时，它的频率应该会下降，但当整个雏鸟恢复时，它的频率会回到基线水平。

In many kinds of animals, parental care is essential for the survival of newborn progeny, yet it is sometimes terminated by the parent long before the offspring reach full independence. The ecological causes for such desertion are diverse, but center on the evolutionary principle of maximizing reproductive success. In other words, parents may be able to raise more young in the long run by truncating investment when the cause is already lost (for example, during a drought, if the food base deteriorates dramatically). Recent field studies of birds (egrets and herons) have identified another condition under which parents appear to abandon already-hatched young. When brood size drops from three or four chicks to just one surviving nestling, parents may forsake it and use the remainder of the summer breeding season to renest, perhaps thereby producing a larger brood. The signal for such a switch in parental behavior is presumably the number of chicks in the nest. This idea has been supported by observational data on two species, but has not been tested experimentally. The present study aims to fill that void with a set of complementary field experiments, using an extremely common species (the cattle egret). In the first experiment, brood size will be artificially reduced from three chicks to just one, by transferring the newly hatched nestmates to undersized broods nearby, where they are accepted readily. Controls will consist of broods whose young are handled the same amount, but then returned to their own nest. Dr. Mock predicts that parents will desert the singletons more frequently than the control broods. In order to explore whether this effect decreases as the remaining amount of breeding season wanes, the first experiment will be run three times throughout the nesting period, with the prediction that the later singletons are less likely to be abandoned than those occurring early in the season. If so, parents would have to take both number of chicks and time of season into account. The final experiment involves the behavior of the nestlings themselves. In this and related species, chicks hatch at different times, and the slightly older and larger nestlings persecute their younger siblings, even to the point of killing them. If becoming a singleton is a perilous condition (increasing the risk of parental desertion), then chicks may reduce their rate of fighting siblings when brood size drops from three to two. This will be tested by removing the youngest of three chicks from nests kept under continuous observation and then replacing it three days later. If aggression depends on the number of nestmates, its frequency should dip when only two chicks are present, but return to baseline levels when the full brood is restored.