What should I do if I've found a nursing mother and/or kittens?
If spot a cat who appears to have swollen nipples, then she’s likely a nursing mother cat, and her babies are probably hiding nearby. It is imperative for you to catch the mother cat all her babies as well since it’s a death sentence for the kittens if they are separated from their mother for too long and they are still nursing on her.
Depending on whether you have caught the mother cat or kittens first, the approach to find the rest of the family will differ.
If you have the kittens but not the mother
If you find baby kittens, you can use them to lure and trap the mother. Catch all the babies you can using a trap and food or simply pick them up if they are still very young and immobile, then place them in a carrier. Once you have all the babies in a closed carrier, push the door against the back of a trap and engage the trigger mechanism.
By doing this, the mother cat will have to walk into the trap in order to reach her babies located at the back. Their cries will usually be enough to make her instinctively enter the trap. Cover both the carrier and the trap with a towel so the mother cat must enter the trap to see her babies.
If you have the mother but not the kittens
To find a mother cat’s babies, walk around the area with the mother in the trap/carrier and let her cry out for them. Hearing the sound of their mother may trigger them to mewl in return, making it easier for you to locate them. Once you’ve found them, either catch them with food and a trap or pick them up if they’re young and immobile.
Once you catch the whole family, keep them together, schedule the mother for spaying, and socialize the babies for rehoming. If the mother is friendly, a home can also be found for her as well. Mother cats are capable of nursing her babies even after being spayed, so it's smart to have her fixed right away.
Caring for nursing kittens is much easier with a mother cat around, so make sure you keep them together until the kittens are at least 8 weeks old and reliably weaned. If the kittens are orphaned or you cannot catch the mother cat, then you will need to care for the kittens in her stead.
Only trap mother cats and babies if you have the resources and capacity to care for them! If you are not capable of taking them on, focus on interacting with the kittens and socializing them while actively searching for individuals/rescues who can provide the family with adequate care.