I feel for you.
How big is your onion patch? Maybe not much help but in the old days my dad used to stretch black cotton (sewing cotton) between sticks crisscrossing his lettuces etc to keep the birds off. It worked. About 10 cm high and lower at edges to stop them sneaking underneath.
If it’s not too large, maybe just surrounding the perimeter with a barrier may stop them wanting to land in a restricted space. If nothing else to hand and as it’s short-term possibly even several sheets of newspaper hung over string and clipped together with clothes pegs/bottoms buried could work if it’s not too wet and windy. (I haven’t tried it)
Finally, as a last resort, as blackbirds are the likely culprit you could always take the nursery rhyme route!
(Only joking)
Enjoy your shallots.
Jan
