translation.en: I wanted to know about their seasonality, what time of year, this is when they first arrive, but how long are they around for? Does that give any clues?
translation.zh: 我想知道它们的季节性， 我们知道他们每年开始出现的时间， 但是它们会呆多久？它们的数目会不会提供一点线索？