Why did a thin red rod produce a stronger pecking response in herring gull chicks than a real gull bill?

The thin red rod produced a stronger pecking response in herring gull chicks because it functioned as a supernormal stimulus.1

In ethological studies, researchers found that while a parent gull's bill naturally elicits food-begging behavior in chicks, the response is guided by specific features, such as the red spot on the bill, the contrast between the spot and the rest of the bill, the thinness of the bill, and its movement. By experimentally manipulating these features, scientists like Nikolaas Tinbergen discovered that they could create an artificial object—a thin red rod with three white rings near its tip—that exaggerated these key characteristics.1

Because this artificial model possessed these features in a more pronounced or concentrated way than a real gull's bill, it elicited a more intense instinctive response from the chicks. This phenomenon demonstrates that nature can sometimes be "improved upon" by creating stimuli that trigger instinctive behaviors more effectively than the natural objects they evolved to recognize.1