I read these parrots die not so much during the sea crossing, as during their wintering in coastal Australia. There something could be done, perhaps with supplementary feeding or introduced predators.
Another thing would be trapping and poisoning the introduced predators in Tasmania.
I think after decades of study of these guys - their decline has been researched since the 1980s at least - no one has found the silver bullet 'cause'. Moreover it's likely the classic extinction vortex of multiple things acting in concert such that no single things drives the decline (or potentially, recovery)
My point about Bass Strait is that it used to be a land bridge and now is is ocean! So they didn't have to island hop over water so much, which is is big ask for a small parrot.
There has been significant habitat loss on the wintering grounds.
The remaining population has adapted to feed on some weed species to some extent, but whether that is sufficient for them to meet physiological needs for the migration home is another matter. In this limited remaining habitat there is competition from introduced seed eaters - finches and mice. There is however supplementary feeding, although I've not seen any data on whether that has made a difference.
There has also been significant habitat decline on the breeding grounds, where fire regimes that existed for thousands of years stopped being practiced in more recent history.
Their is competition for nest holes from introduced species (including bees, and a native species of glider, introduced to Tasmania), as well as predation by foxes / cats both in the breeding grounds and wintering grounds. Again these are being addressed to varying extents - but whether the actions taken have made a significant difference is debatable.
They are at risk from disease (PBFD is rife in Australia)
The small population size increases the likelihood of inbreeding depression (in fact the captive breeding program was pretty much restarted a decade ago because their was not enough genetic diversity in the supposed insurance population)
Not trying to dismiss it - I was part of the OBP survey volunteer team for several winters - but I'm just a bit cynical of how much hope can be derived from the pumping of another few dozen adult and juvie birds into the population in the hope some of them miraculously survive the multitude of only partially abated threats that have wiped out most of the natural born birds
