鸽子远距离飞行不会迷失方向．其原因是鸽子根据什么来辨别方向？

Chen Wei isn't at school today ________ he is taking a robot competition in Shanghai.

阅读诗歌，回答问题。

                                                                                       塞上听吹笛
                                                                                      （唐）高适
                                                           雪净胡天牧马还，月明羌笛戍楼间。
                                                          借问梅花何处落①？风吹一夜满关山。
                                                【注】①梅花何处落：是将曲调《梅花落》拆用。