The red spot

The red spot on a herring gull's beak has played an important, but controversial role in the study of animal behaviour.   John Whitfield writing on the Nature website in 2009 explained:

"One of the most famous experiments in biology isn't the solid piece of work it's usually portrayed as, say Dutch researchers who have replicated the study. Instead, it's more like an anecdote that became slightly more legendary each time its author retold the story.
The work in question was done in 1947 by the Dutch researcher Niko Tinbergen on the begging behaviour of herring-gull chicks. At the time, the dominant idea in animal behaviour was that learning was all-important. Tinbergen argued that animals come into the world with instincts already adapted to their environments.
Adult gulls have a red spot on their lower bill. Tinbergen, who shared the Nobel Prize for Physiology or Medicine in 1973, presented wild chicks with model birds bearing spots and measured how much they pecked at the model.
The story that made it into the textbooks is that chicks have a powerful innate tendency to peck at red dots, which has evolved as a way of getting their parents to feed them. The original paper, however, shows that Tinbergen found that chicks actually pecked more at a black dot than a red one.
In a follow-up paper written in 1949, Tinbergen concluded that this strange finding resulted from a mistake in his methods. He had tested red, black, blue, white and yellow spots, but he presented the 'natural' red spot much more often than any other. The chicks, he decided, became habituated to the red spot and stopped pecking at it.
Tinbergen never fully tested this idea. Instead, he did another experiment comparing red and black spots presented equally, and found that the red was indeed preferred. From this result he came up with a 'fudge factor' which, when applied to the original data, reduced the chicks' apparent preference for colours other than red. He was initially explicit about this, but by the time of his books The Study of Instinct (1951) and The Herring Gull's World (1953), he had stopped mentioning this correction, and presented the finding as if it were based on unmodified data.
"The changes from one paper to the next aren't that substantial, but if you compare the first and the last, a lot of small steps make a big one," says Carel Ten Cate of Leiden University in the Netherlands, who has analysed and repeated Tinbergen's work. "There were definitely some changes to make the result more simple and clear cut than it initially was."
Tinbergen did other experiments with gull chicks — showing for example, that they will peck even at a disembodied red spot on a stick — so he may have felt he had made his point. He may also have simplified his results for rhetorical purposes.
Ten Cate and his colleagues redid Tinbergen's original experiment and got the same result — black was more attractive than red. They also did the experiment he never did, presenting each colour equally often, and found that Tinbergen's intuition had been correct after all — the birds tended to peck more often at red spots."