When you find a litter of kittens with no mother cat in sight, do not assume the litter has been abandoned. The mother frequently leaves her litter in order to find food and water, and can be gone for hours at a time. This is normal behavior for all animals.
Collecting kittens and taking them to the shelter is never the best solution. Most shelters in the US will kill those kittens instantly because they do not have the setup to handle neonatal kittens. Plus, very young kittens must be with their mothers during their first few weeks of life in order to survive.
Please leave litters where they are. Mom will be back soon! And if you are really worried, check back in a few hours – most likely mom will either be back by then or the litter will have been moved.
Happy Spring aka Kitten Season!