caterpillars of swallow tail

A few weeks ago, 27 of july. I posted a photo of the swallowtail butterfly,
who was sitting around on the foeniculum plant.. .

Today I noticed some peculiar caterpillars on the plant.
and to my surprise they are the caterpillars of this beautiful butterfly!

Would that be great to watch the caterpillars grow... go into little cocoons
and watch them come out one day?
But how can I save them, from being lovely snacks for the birds?
This struggle for survival will be not so easy...
maybe someone knows?????
Advise will be very welcome.!