Question

a large quantity of birds 后面的谓语动词是单数还是复数

Answer 1

a quantity of+可数名词复数、谓语动词用复数;+不可数名词、谓语动词用单数.即修饰可数或不可数名词均可,作主语时采取就近一致的原则,其谓语动词通常与短语中of后面的名词的数保持一致.
例如:
A large quantity of beer was sold out.大量的啤酒被售出.
A large quantity of blouses were on sale.大量罩衫有售.

Answer 2

当然是复数，主语是birds,
a large quantity of 是定语，adj. 大量的
所以谓语是和birds相连的

Answer 3

把a large quantity of的作用看成much或many 即可。
所以，a large quantity of birds 后面的谓语动词是单数还是复数，看birds即可。