We saw this bird sunbathing in our garden last month. It stayed like this for a few minutes and seemed enjoying the sun. Then it flew to the woods our garden backed onto.
Hi Tomoko, looks like a female blackbird carrying out what is referred to as "Sunning" which is part of many birds essential routine which helps maintain healthy plumage, the birds add preening oil to feathers which also helps rid them of any parasites. You can read more on the highlighted Sunning link.
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Regards, Hazel