Juvenile? This bird was singing some of the time I watched it. It was similar to a chaffinch song but not the same. It was also similar to a wren's song - but without the trill in the middle. Beak looks wren-like to me but not the tail. Which makes me think it may be a juvenile.
Beira do Río Cabe ao pasó por Monforte de Lemos/ Excrementos en río Sil A Covela