Hi Rachel, If, as you say, the bird looks healthy, can run and fly, then don't worry about the Red-legged Partridge being in your garden I'm sure its absolutely fine and will eventually pair up for breeding and will go off in search of a mate. We've also had them in our garden, sometimes one on its own and then the pair of them although we are less than quarter mile away from fields. I also put some mixed seeds down for them; they are beautiful birds with lovely markings so enjoy your visitor.
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Regards, Hazel