For a week now, we’ve spotted a little bird (possibly starling) hopping about & fluttering up & down on an upstairs windowsill of a house opposite. It stays there all day & only flies off at intervals for a minute or so & at dusk flies off.
It will be seeing a reflection and wasting energy trying to compete with itself. Ideally, the homeowners need to put up blinds or some other solution to remove the reflection.