Learning how to build a nest plays an important part in the breeding success of birds. For example, Dr. Snow has recorded the success of a number of blackbirds in several successive years. He finds that birds nesting for the first time are less successful in breeding than are older birds, and also less successful than they themselves are a year later. This cannot be a mere matter of size and strength, since blackbirds, like the great majority of birds, are fully grown when they leave the nest. It is difficult to avoid the conclusion that they benefit by their nesting experience. Which one of the following, if true, would most weaken the argument? ['Up to 25 percent of all birds are killed by predators before they start to nest.', 'The breeding success of birds nesting for the second time is greater than that of birds nesting for the first time.', 'Blackbirds build better nests than other birds.', 'The capacity of blackbirds to lay viable eggs increases with each successive trial during the first few years of reproduction.']