I don't believe the newts feed on tadpoles, but they do feed on the eggs before they hatch. Oftentimes in early summer, you'll see some still water on the sides of streams where water has been captured during periods of water high water. Frogs lay their eggs in these patches of still water (I guess in the hopes that the stream doesn't flood and wash their eggs away), and red-spotted newts will usually just take up residence in the pool next to the eggs getting fat off of them. I always have trouble finding red-spotted newts at other times of the year, but in early summer, they're almost always found feasting on frog eggs.
