A singing Isabelline Warbler (Hippolais opaca) doubtless was the highlight of today's trip around Faro. We found the bird near the hide at the Quinta do Lago Golf-course this morning, where it has been singing out of the dense crowns of the Umbrella-Pines at this place. After a while we also got views of this bird climbing through the dense vegetation and sitting in the open every now and then just for a moment. The bird lacked any yellowish or greenish color-tone of the possible confusion-species Melodius Warbler (Hippolais polyglotta). The song also differs - it resembles a bit a Reed Warbler (Acrocephalus scirpaceus) which does not sing out of the crowns of Pine-Trees and shows some reddish-brown in the plumage.
João Tiago Rocha Tavares managed to get the following photos of the bird:
A link to an external site on the ID of Melodius Warbler versus Olivaceous, including sound files, is here.
This his apparently a bird on spring migration - they are very late migrants.
There is only one known breeding-record for Portugal over the last years (Alentejo). This bird is rare - but also easy to overlook, and ID in the field is very difficult, especially when the birds don't sing (autumn-migration).
Links about the splits of Hippolais-Warblers are here and here.
This is also interesting.