Why might a nesting gull prefer a large artificial egg to its own eggs?

A nesting gull may prefer a large artificial egg to its own because the dummy egg acts as a supernormal stimulus.1

In the study of animal behaviour, supernormal stimuli are artificial objects that exaggerate the features of a natural stimulus, triggering a stronger instinctive response than the natural object itself. For example, when a nesting herring gull or oystercatcher is presented with a choice between its own clutch and a dummy egg five times the size of a real one, it will select the oversized dummy, even though the bird cannot physically sit on it in the normal way. Similarly, a black-headed gull may prefer a clutch of five eggs over its own clutch of three.1

These responses demonstrate that instinctive behaviors are often guided by specific, predetermined sensory cues rather than an internal recognition of what is "correct" for the species. When these cues are exaggerated, the animal's instinct is driven to an evolutionary extreme, prioritizing the supernormal object over the natural one.1