Birds of various species, from pigeons to swallows to larger birds, ca

Birds of various species, from pigeons to swallows to larger birds, can navigate long distances on Earth, across continents and hemispheres. That they can traverse these distances thanks to a magnetic sense has been demonstrated through tests in which birds fitted with magnets have lost their navigational capability. Precisely what biological mechanism enables birds to orient in this way is still something of a mystery, however, with two theories prevailing.

One theory is that birds possess magnetic sensors in the form of grains of magnetite, which is an easily magnetized form of iron oxide. Such magnetite grains are common not only in animals but even in bacteria, where they have been established as a component enabling magnetic orientation. In the case of birds, magnetite grains are numerous in beaks, as dissections of pigeons have confirmed. Moreover, in another experiment, the trigeminal nerve, which connects the beak to the brain, was severed in reed warblers; the affected birds lost their sense of magnetic dip, which is critical to navigation.

Critics of the theory have pointed out that the abundance of grains in the beak are not concentrated, as would be expected in a sensory organ, but rather found in wandering macrophages. And while an alternative explanation for birds' sensory abilities might posit magnetite grains outside of the beak, such an explanation would be supported neither by the beak dissections nor by the tests of severed trigeminals. Critics of magnetite-centric theories suggest a second theory: that the magnetic field of the Earth has an influence on a chemical reaction in birds, specifically in a bird's retina. Experiments have demonstrated that destroying the portion of a robin's retina known as cluster N eliminates the bird's ability to detect north. Birds' eyes do not contain magnetite grains, however. Rather, some advocates of the theory that birds navigate by retinal interaction believe that a retinal protein known as cryptochrome processes magnetic information within the cluster N. Surprisingly, the mechanism by which cryptochrome could detect magnetic orientation depends on quantum mechanics: when hit by light, the cryptochrome would create a pair of particles, one of which subsequently presents information to the eye, in the form of a spot, when it is triggered a corresponding particle after that particle has traveled some distance.

1. According to the author, critics of the first theory presented in the passage believe which of the following to be a factor enabling a migrating bird to navigate?

A. Wandering macrophages containing magnetites
B. The abundance of grains in the beak
C. The function of the trigimenal nerve
D. The function of the portion of the retina known as cluster N
E. The possibility of magnetite grains in parts of the bird other than the beak

2. According to the passage, both theories described by the author have been developed to explain how

A. birds can navigate across oceans without the use of landmarks
B. birds use magnetism to navigate long distances
C. birds of various species navigate differently
D. birds fitted with magnets have lost their navigational ability
E. birds are able to navigate long distances using magnetites

3. Which of the following would be most useful to know in order to evaluate the retinal theory of navigation?

A. whether the absence of magnetites in the retina is the reason that robins fail to detect north when their cluster N are destroyed
B. whether other species' retinas are identical to the retinas of robins
C. whether quantum mechanics is a popularly understood theory
D. whether cryptochrome processes magnetic information
E. whether cluster N is found in wandering macrophages

4. The author is primarily concerned with

A. discussing different theories
B. analyzing different techniques
C. defending two hypotheses
D. refuting established beliefs
E. revealing new evidence