Northern New England's annual amphibian migration is always perilous, but critters that cross roads to breed are facing an additional challenge this year: a delayed start after the long winter. Every spring, several species of salamanders and frogs travel to vernal pools —temporary bodies of water created by melted snow—to mate and lay eggs, and the resulting offspring need several months to develop and grow legs before the pools dry up in summer. Wildlife officials say the migration is running a week or two behind this year, cutting into that critical development time. That could affect millions of animals across Vermont, Maine, and New Hampshire, says Eric Orff, a wildlife biologist with the National Wildlife Federation.