I didn't realize that about natural sunlight. The eggs I found were in fact in a shadier spot in the woods, where they only got dappled, moderate sunlight filtered through the trees. Reminds me of a book I read, The Vanishing Frogs. It brought up the threat of UV rays passing through a diminished ozone layer as a possible cause of frog decline. Much less obvious than human-caused destruction, introduced predators, etc.