A female kitten can enter heat as early as 4 months of age. She’ll try to get outside at every opportunity, some have even been known to break through window screens or almost magically slip out open doors like lightening!
When daylight hours and temperatures increase, a female cat may enter heat every 2-3 weeks.
Spay and Neuter surgeries are suggested before the animal is 6 months old. Some vets will perform the surgery if the animal is at least 2 lbs, others prefer waiting until the animal reaches 4 lbs. It will depend on the facility and the animal (some cat breeds will reach sexual maturity before they reach 4 lbs).
There are options for people doing the right thing like you are. It's really vital that these little strays are spayed and neutered.
Don’t forget to call your local Humane Society for guidance as well. If you have a Pet Smart in town, they often have the numbers for rescue organizations that may also help, at least with recommendations for where (or who ) to go.
Keep up the good work