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The 26-year-old all-rounder has spent 12 years with the Spirit, but decided the time was right for a change.
The massage therapist moved to Christchurch three days before lockdown last year.
She considered playing for Canterbury.
But the prospect of linking up with the White Ferns trio Suzie Bates, Katey Martin and Hayley Jensen, and working with coach Craig Cumming, was an appealing idea.
"I’ve already had some great chats with him [Cumming] and he is going to be fantastic to work with," she said.
"Basically, I’m just looking forward to getting started. I’m very excited about being in a fresh environment.
"I think it will be an exciting team to be involved with during the next few years."
Leydon-Davis is hoping the move well help rejuvenate her career.
She played the last of her eight T20s for New Zealand in 2016 and made one appearance in the ODI side in 2014.
But a back complaint slowed her progress and she has had to reinvent herself as a batswoman who can bowl a little.
"I got a disc bulge and had a couple of surgeries and have basically been working my way back from that during the last five years."
Leydon-Davis hails from a sporty family.
She has represented New Zealand at badminton.
Her brother, Oliver Leydon-Davis, and sister, Susannah Leydon-Davis, teamed up together for the 2018 Commonwealth Games on the Gold Coast.
Oliver has been to three Commonwealth Games and is chasing a spot in the Olympic team.
"I do like to remind him now and then how I bowled him out in the back garden," she said.
Leydon-Davis stepped back from elite badminton in 2015, but still coaches.
In her only ODI appearance for the White Ferns she nabbed a five-wicket bag.
With an effort like that you would expect a least one encore appearance.
"You tell me," she replied when asked why she did not get another opportunity.
"Obviously, it was a very exciting time against the West Indies here [at Lincoln].
"But, unfortunately, I was not selected for another one.
"That is the nature of the beast at times."
Otago Cricket Association high performance manager Simon Forde it was "heartening that players of the quality of Felicity" see the Sparks as an opportunity to develop their cricket.
"It is a credit to Craig Cumming as head coach, the support staff and our players for developing a strong culture where growing players on and off the field is our number one priority," Forde said.