How did poisons become the preferred means of predatory snakes to kill mammals and birds?

First off: THERE IS NO SUCH THING AS AN EVOLUTIONARY SCALE OF COMPLEXITY. The impression such the beings bottle be ordered from bacteria on human belongs customary the 18th century.
There were both birds and mammals about in the Mesozoic Age, when snakes evolved. And the first snakes were homogeneous to boas, not vindictive ones; the colubroids (the group that includes the vindictive forms) are first recorded in the Miocene (that is Tertiary, long postern the diversification of modern birds and mammals). Therefore, you are origin coming out of a false premise.
Also, salivary glands are present customary all tetrapods, and especially in amniotes (reptiles, game and mammals). Saliva control enzymes, and it's not that rambling (from a biochemical or evolutionary viewpoint) such all mutations that changed those enzymes into other more toxic ones were retained considering connected with their advantage albeit subduing prey. Indeed, the evolution of vindictive saliva did not happen once: Gila monsters also at entrylevel some insectivorous mammals also enjoy similar (but independently evolved) systems.
Thus:
- snakes go for it not go before birds and mammals
- poisons are made connected with enzymes, and integer enzymes can be "poison" to some living being or other (ask the bacteria that are prohibit by the saliva in your mouth!)