I think they weaken faster than the primary feathers, so if the instrument is for your own use I reckon the only problem would be the time you lose on them before finding ones that are strong enough.

In Ireland and possibly elsewhere in the Northern hemisphere, this is the time of year when rooks, crows etc. shed flight feathers which you can find on the ground while out walking near rookeries. The swan moult (I get them at low tide in a river estuary) is slightly earlier. A possible inscription for a harpsichord lid would be "no birds were harmed in the production of this instrument". Or a haiku to that effect. In Latin, perhaps.
Happy quilling, Michael



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