u cant catch the legendary birds in pokemon crystal except for lugia and ho-oh to catch
lugia u must get a silver wing from an old man at viridian city in kanto for ho-oh u must
catch all legendary dogs and then talk to the guy in curealean city's pokemon center than
he will give u something after that go to the tower where you found suicune in and ho-oh
should be there

i agree with him

To get them you should trade from green,red,blue or yellow versions to get the birds.