The lecturer states that, although+【閱讀觀點(diǎn)】 the commonly accepted three theories of birds’ navigation sounds convincing, it suffers from several flaws on various grounds, as is presented below.

　　The first problem with the navigation theory presented in the text is the lack of substantial evidence[閱讀觀點(diǎn)]. As is stated in the lecture,+聽(tīng)力的細(xì)節(jié) many birds could still navigate at night; therefore they could not navigate by using the sun as compass, since there is no sunlight at night for their reference. And stars do not help either, as there is only one bird species that is known to navigate by using stars. Therefore the sun and the star could not be birds’ compasses.[聽(tīng)力觀點(diǎn)]

　　[閱讀觀點(diǎn)]The second flaw concerns birds’ ability to recognize topographical cues. They can remember a certain journey, but only after their previous travels via the same route. The phenomenon that birds that get lost could easily find their way home serves as a crucial example. On the contrary, the theory presented in the text is questionable.

　　Furthermore, [閱讀觀點(diǎn)]although birds can sense the earth’s magnetic field, the text ignores a crucial fact that birds could only distinguish north and south[聽(tīng)力觀點(diǎn)]. What they could not perform is to find an indefinite destination hidden in complex landscape, which even troubles modern geographical technology, by their simple censoring system. Therefore, birds’ ability to sense magnetic field could not be the explanation for their navigation ability.