We have only seen them on three occasions in Cuba. All have been during the winter months and all were rather fleeting. Until recently there had been no study of White-collared Swift in Cuba but that changed in 2019 when Dr Rosalina Montes Espin published her doctoral thesis on their Reproductive Biology and Conservation which I have only just come across although I knew she was working on it. It's in Spanish so it will take me a while to read!
I also come across this paper regarding the observation of a pair at a waterfall nest site in neighbouring Dominican Republic.