99. Read the following passage, then answer the question below.

Every autumn, billions of birds travel thousands of kilometers to wintering grounds they may never have seen. How they find their way has puzzled naturalists for centuries. Experiments in the twentieth century established that birds use multiple cues: the position of the sun by day, patterns of stars at night, and landmarks such as coastlines. But none of these explains how a young bird, flying alone on its first journey under overcast skies, can still hold its course.

An answer began to emerge in the 1960s, when researchers placed European robins in cages with no view of the sky and observed that the birds hopped in the direction of their migration. The result was unchanged when the cages were shielded from sunlight and stars, yet it vanished when the surrounding magnetic field was altered with coils. The robins, it seemed, possessed an internal compass sensitive to Earth's magnetic field. Later work added a surprising detail: the compass appears to work only in certain wavelengths of light, which led some biologists to propose that the sensor is located in the eye and depends on a light-sensitive protein.

Not every question has been settled. Some scientists believe that a second magnetic sense, based on tiny iron-bearing particles in the beak, allows birds to detect variations in field strength and thus to estimate their position, not merely their heading. Critics counter that the particles found so far may be by-products of ordinary cells, not sensors. Experiments that disrupt the beak's nerve have produced mixed results.

What is clear is that migration relies on layered systems. A bird crossing an ocean may begin by following a magnetic heading, correct it with the sun at sunset, and fine-tune its landing with smell and sight. The redundancy is probably the reason so many species succeed at a journey where a single failure could be fatal.

Why does the author mention the critics and the mixed experimental results?