听力原文

The birds return to the raft with their fish because they've been trained to do so. From the time it first hatched, each of these cormorants has been reared to a life of obedience to its master.
鸟儿们被训练过只能带着鱼儿回到筏子。从出生的那一刻起，鸬鹚们就开始被训练为忠仆。
The birds are, in effect, slaves. But they're not stupid. It's said that cormorants can keep a tally of the fish they catch, at least up to seven.
这些鸟儿是高效的奴隶。但是它们并不是白痴，据说鸬鹚能够记住它们捕获的鱼的数目，至少能记住七条。
So unless they get a reward now and then they simply withdraw their labour. The fishermen, of course, keep the best fish for themselves.
除非它们不时得到打赏，当然这不过是收回一点自己的劳动成果罢了。渔夫理所当然得把最好的鱼留给自己。
The cormorants get the leftover tiddlers. With its collar removed, the bird can at last swallow its prize. Best of all, one it isn't meant to have!
鸬鹚则能享受剩余的部分。项圈摘除后，鸟儿们终于能享用它的战利品了。最美妙的莫过于得到额外的打赏。

重点讲解

重点讲解：
1. in effect
其实；实际上；
eg. That deal would create, in effect, the world's biggest airline.
事实上，这一协议将促成世界最大航空公司的成立。
eg. The two methods were in effect identical.
那两个办法实际上是一样的。
2. now and then
时而；有时；
eg. He takes my sister out to the theatre now and then.
他时常带我妹妹去剧院。
eg. Everyone needs a pat on the back now and then.
每个人都需要不时地得到鼓励。
3. best of all
最好的是；
eg. It was comfortable and cheap: best of all, most of the rent was being paid by two American friends.
房子既舒适又便宜：最妙的是，大部分租金是由两位美国朋友支付的。
eg. Best of all, over time the benefits endure and continue to improve.
最棒的是，这些益处会随着时间持续存在，而且会逐步增加。