Thank you very much for your reply. Sorry for the poor quality of the video,it was taken by a DC,and I am a novice. I found that I always focus to the wrong object especially when the bird is close to leaves. I hope that it is not an Eurasian Tree Sparrow,but it turn out that it is. But it seems that there are not many species of bird in Kowloon Park this year,or may be I need to learn more to discover them in the park as I am a novice. Thanks again.