在一个简单句中：There are some birds...已有动词are,所以不可再用另一个动词,故用非谓语动词,即-ing形式作定语,相当于 There are some birds which are singing...
不过,这个句子虽然正确,但不是好句子,最好直接说,
Some birds are singing in the tree.
所以,一般情况下,最好不说：There be sb/sth + doing sth.纯属多余或啰嗦.
e.g.There were a lot of people watching the accident.不是好句子,应当直接说：
A lot of people were watching the accident.