At first it may seem a trifle odd that we can handle these butterflies until you consider that we rescue them as eggs... Once the eggs hatch we feed & care for the caterpillars from the time they are the size of pinhead until they're as big as your index finger, before they go into their chrysalis stage, until they emerge as butterflies at which time we release them to nectar, mate, and find a host-plant upon which they will deposit their eggs, and begin the cycle over again.
During the 2-3 weeks they are in our care, we're handling them and talking to them as we clean their enclosures and feed them several times a day, so they get very used to us, our touch, and the sound of our voice.
Each species of butterfly lays it's eggs on a specific species of plant that their larvae can feed on as caterpillars until they morph into butterflies.
At every stage of this 2-3 week process, they are vulnerable to ants, spiders, mantises, and birds. In nature you'd get one mature butterfly out of every hundred eggs laid.
We grow many of the host species for various species of butterflies in our gardens to increase the likelihood of them locating the specific plant they need when the time comes.
By checking the plants every few days, we're often able to take the eggs into "protective custody" we increase the chances of reaching maturity and flying off to perpetuate their species as much as 95-100%.
If some of them deposit some eggs before moving on it accounts for about 1/10 of what they'll lay, but as I said, that 10% will yield nearly 100% of success.
It's a strong possibility...especially if there are a lot of wild areas that are left to support their HUGE, OBVIOUS, UGLY Host Plant - the Milk Weed - that folks spot & makes it a point to eradicate, not understanding (or perhaps caring) the role it plays in the miraculous thousand-mile migration that we're now on the brink of losing ...forever...
Truth is, those Black Swallowtails use PARSLEY as their main host plant. Pretty much every gardener freaks-out when they find "big ugly worms" chowing-down on their precious parsley!...and their natural response is "KILL 'EM!" ...without connecting the dots that those "Ugly Worms" are a necessary part of the life-cycle of The Black Swallowtail.
...but I'm afraid, it's worser than that...
"Ugly Worms" that eat all sorts of crops have fostered a huge industry of "Worm/Weed Killing"... not only through the (over)use of toxic pesticides but also have gone as far as to genetically engineer toxins directly into the DNA of the plants themselves.
Monsanto & Bayer are very very good at eradicating "Ugly Worms"... in order to keep worms out of peaches, corn & parsley, they've all but eradicated butterflies ...(and collaterally) honey bees. as well.