That should be easy to test, by leaving a few of the same harnesses within reach of the birds. If the birds pick up the stray harnesses too, it's likely object

That should be easy to test, by leaving a few of the same harnesses within reach of the birds. If the birds pick up the stray harnesses too, it's likely object attraction. If the birds leave those alone or merely attack them (like picking them up and dropping them from some height), it's empathy. In any case, it's not altruism unless the birds take on a risk themselves by trying to remove it from others -- it's merely social behaviour.