I want to know the answer to the following question:
How many bird species are obligate parasites?
All the background I'm provided with is that "Brood parasites may be either obligate brood parasites, which must lay their eggs in the nests of other species because they are incapable of raising their own young, or non-obligate brood parasites, which sometimes lay eggs in the nests of conspecifics to increase their reproductive output even though they could have raised their own young.". Is that enough to answer the question?
Answer: no