Proximate Causes of Parental Desertion

在许多动物中，父母的照顾对它们的生存至关重要。 新生的后代，但它有时是由父母长期终止 在后代完全独立之前 的生态原因 这种遗弃是多种多样的，但集中在进化的原则， 最大化繁殖成功率 换句话说，父母可以 从长远来看，当事业失败时， 已经失去了（例如，在干旱期间，如果粮食基础 急剧恶化）。 最近对鸟类（白鹭和苍鹭）的实地研究发现， 另一种情况下，父母似乎放弃已经孵化的 年轻 当一窝雏鸟的数量从三四只减少到一只时 幸存的雏鸟，父母可能会放弃它，并使用剩余的 夏季繁殖季节，也许从而产生一个更大的 育雏。 这种父母行为转变的信号大概是 巢中的雏鸟数量 这一想法得到了 两个物种的观测数据，但尚未经过测试 实验性的 本研究的目的是填补这一空白， 补充现场实验，使用一个非常常见的物种（ 牛背鹭）。 在第一个实验中，育雏的大小将被人为地减少， 三只雏鸟变成一只，把刚孵化出来的巢友转移到 在附近的小巢里，它们很容易被接受。 控制 将包括育雏其年轻的处理相同的数额，但然后 回到了自己的巢穴。 莫克博士预测， 单胎比对照组更频繁地孵化。 为了探索这种影响是否会随着剩余的 繁殖季节的数量减少，第一次实验将运行三个 时间在整个筑巢期间，与预测，后者 独生子女被遗弃的可能性比那些发生在早期 赛季 如果是这样的话，父母们就必须把两个数量的小鸡， 考虑到季节。 最后一个实验涉及雏鸟本身的行为。 在这个物种和相关物种中，小鸡在不同的时间孵化， 稍大和稍大的雏鸟陪伴着它们的弟弟妹妹， 甚至不惜杀死他们 如果成为一个单身是一个 危险状况（增加父母遗弃的风险），然后 当雏鸟数量减少时， 从三点到两点 这将通过移除最年轻的 三只雏鸟从巢中被连续观察， 三天后更换。 如果侵略取决于 当只有两只雏鸟出现时， 但当全部育雏恢复时，恢复到基线水平。

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.