Let's not forget, snakes are MAJOR predators of baby birds today, even for species with biparental care.Parental care =/= total immunity of the babies from all attacks!!
Excellent point. Never underestimate the stealth abilities of snakes. Many small mammals lose their babies to snakes, as well. More generally, infant mortality to predation is often exceptionally high even in species with long-term biparental care.
Cheers, -Mike H.