Helping verbs are verbs that help the main verb in a sentence by extending its meaning. They can also add detail to how time is conveyed in a sentence. As a result, helping verbs are used to create the most complicated verb tenses in English: the progressive and the perfect aspects.
When migrating, birds usually migrate when there's not enough food, shorting of daylight, and when it's getting colder in winter. Birds that migrate do so in the late summer through the fall and in the late winter through the spring. Birds and use a radical pair-based compass to "see" the Earth's magnetic field when migrating.
