Answer: It's natural for plants to lose their older leaves and the oldest leaves of your Donkey's Tail are located at the crown of the plant. Over time the fleshy leaves will wither and die, leaving only a bare stem with a leaf scar. To keep the plants full, you can pinch out the growing tips, which will force the crown to develop new stems.
If birds are in fact eating the leaves of your plant (which is certainly a possibility, but not something I've ever heard of before), you can place bird netting or hardware cloth over the crown of the plant to prevent easy access.
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