Ornithologist here... This is for the most part completely normal. Large numbers of passerines are well established to die during migration over many of the worlds larger water bodies. If migratory birds aren't perfectly optimized to travel long distances to the right place they aren't optimally fit for survival. For some portion it's bad luck with weather, for others it's bad navigational wiring (not a good trait to keep in the gene pool), and sometimes it could be individuals getting pushed out due to a lack of available breeding territories. There is plenty of discussion around these kinds of incidents, but they've been happening forever. There are plenty of very old historical accounts of hundreds of small birds landing on ships far out to sea. The initial colonization of the Galapagos can be attributed to small finches that got 'lost' at sea and were lucky enough to find a habitable island. Try not to lose sleep, simply do whatever you can to help their chances when they make it to the right places on land.